Circus raises a lot of good points. I'll go one step further. Rather than limiting yourself to what you can see in one day from SJ, why don't you book your first night or 2 (if not more) entirely outside the central valley? That way you'll get to see places you simply can't really visit from SJ in just one day. Arenal, Monteverde and MA are the most popular choices but you could also try Tortuguero, Cahuita, Osa Peninsula, Dominical, Montezuma, Tamarindo etc. Did you notice I deliberately left Jaco out of that list? Okay, at least Jaco gets you out of the Gulch and yes they have mongering but that's about all you can say in favor of it. There are many much nicer places to go and for many mongers Jaco has become just another been there done that place which features mongering that you could just as well stay in SJ for. So you may not be able to do any mongering in those other areas on that part of your trip (unless you arrange for one of your favoritas to accompany you before you go), but there will be plenty of time to make up for that when you finally do get back to SJ for the balance of your trip. Or you could save the travelling around the country for the middle or end of your trip. A day or two outside SJ in the middle is actually a good way to let you recharge your batteries before going in for another round of sexual bingeing.
If I haven't sold you on broadening your horizons by getting out of SJ altogether, then here are my suggestions for day trips from SJ. First of all, I have to say rafting is my personal favorite and it also happens to have a lot of repeat value. Besides, there is more than one section of river to try out. So if you've done that before who says you can't do it again.
Taking buses is a good way for someone either on a tight budget to do things cheaply or someone with experience who has already seen and done a lot (like dapanz and RBC) to knock off targeted attractions. However, IMHO, if you haven't already tried most of the tour options around SJ, as you seem not to have, and you can afford the somewhat added cost of an organized group tour, I think you'll probably be better off starting out with those. For example, you can take a public bus to go see Poas Volcano, but it'll take the better part of the day and that's all you'll see OR you could pay more and go on a group tour that will pick you up at your hotel take you to Poas (and perhaps a few other attractions) and still get you back to SJ in the early afternoon in time for some MP visits or whatever else you want to do.
Next time you might try doing a search as there have been many threads discussing day trips out of SJ in CRT's Non-Gulch forum. Here is one link where I mention the catamarran tour, 4-in-1, the various combinations of Cartago/Irazu as mentioned above plus canopy tours and mountain biking (which no one mentioned):
http://costaricaticas.com/phpBB2/viewtopic.php?p=59542
Here is another one where I complain about another newbie not doing a proper search (as well as other day trip info):
http://costaricaticas.com/phpBB2/viewtopic.php?p=45003&highlight=trips#45003 which in turn included these links:
http://www.costaricaticas.com/phpBB2/viewtopic.php?t=4938
http://www.costaricaticas.com/phpBB2/viewtopic.php?t=4959 (focusing on volcano tours)
http://www.costaricaticas.com/phpBB2/viewtopic.php?t=4850 (focusing on canopy tours)
If you try the search function you will find what you need and you could focus on one or more of the tours that have been mentioned to target your search even further.
Here is my exhaustive, but not even neccesarily complete list and in no particular order:
1) Poas Volcano
2) Irazu Volcano
3) La Paz Waterfall Park (aka by others that have been there in this thread as "that waterfall place")
http://www.waterfallgardens.com
4) Cafe Britt (with I believe 5 different flavors of trips)
http://www.cafebritt.com/CoffeeTour/Index.cfm?
5) Doka Estates (another coffee finca and processing plant)
http://www.dokaestate.com
6) the Butterfly Farm (a real FARM not to be confused with the butterfly GARDEN that you'll find at La Paz)
http://www.butterflyfarm.co.cr
7) Zoo Ave (a nice zoo and aviary out by the airport, the one downtown is NOT recommended)
http://www.zooave.org/ (in spanish but you'll get the idea)
8 ) the Aerial Tram at Braulio Carrillo (for real wildlife viewing as opposed to the adrenaline ride you get on a zip-line)
http://www.rainforesttram.com
9) the Basillica at Cartago
10) Lankester Gardens
http://www.jardinbotanicolankester.org/ing/index.html (for if you want to do it independently, but I strongly recommend you do it as part of a tour that you can simply book at your hotel tour desk)
11) scenic Orosi Valley
12) the pre-colombian indian ruins at Guayabo
13) Reventazon class III w/w rafting
14) Pacuare class III-IV w/w rafting
15) Pascua class IV w/w rafting
16) Sarapiqui class I-II float trip
17) Mountain biking Irazu and Turrialba
http://www.bikingincostarica.com
18 ) Bungee jumping
http://www.bungee.co.cr
19) Horseback riding
20) ATV riding
http://www.atvtourscostarica.com
21) Zip-line (btw, haven't tried it yet but
http://www.colinasdelpoas.com/ingles/index.htm seems to be the closest to SJ and has gotten good reviews)
22) any one of the 3-5 major museums in SJ (including the Gold, Jade and CR Art Museums, the CR Gallery of Contemporay Art and my favorite the National Museum)
23) the Tortuga Island catamarran cruise
http://www.calypsotours.com
24) Offshore fishing near Jaco
http://www.costaricafishingcharters.com
25) Arenal (these last 3 make very long day trips)
26) oxcart factory and craft shopping at Sarchi
27) checking out the national university (co-eds) in Heredia
28 ) Hiring a driver to take you on a scenic ride or to visit one of the above mentioned sites in list above
29) Golfing at the Cariari or Valle del Sol golf courses (actually Cariari may be members only but VdS isn't
http://www.vallesol.com/course.htm
30) the train ride down to the coast (has anyone here tried that yet?)
http://www.ticotraintour.com....
31) Pueblo Antiguo - a touristy recreation of turn of the century Costa Rica
http://www.1costaricalink.com/eng/tours/tours/tempt-pueblo.htm
32) SJ walking tour (don't bother paying for an organized tour, just do it on your own with a good guidebook)
33) Turu BaRi Tropical Park
http://www.turubari.com
34) Ojo de Agua (tel. 441-2808), a water park on the road between the airport and San Antonio de Belén
35) INBio Park, an eco-theme park
http://www.inbio.ac.cr/EN/inbioparque/es/index_eng.html
Haven't added any in a while. How about these:
36) Along the same lines as #34 above but without getting wet, an amusement park in La Uruca which could be a great place to take your novia y sus ninos
http://www.parquediversiones.com/home.html (suggested by MrPublic)
The following bike tours are entirely or mostly downhill and thus possible for even those in less than perfect shape (unlike possibly #17 above). They can be INCREDIBLY exciting and fun but they can also be kinda pricey and may be subject to having enough people signed up:
37) All downhill (aka gravity-assisted) biking starting atop Poas Volcano and including a Doka Coffee Tour (rated beginner/easy)
http://www.lava-tours.com/Day/dayb.htm (also several other tougher rated bike tours)
38 ) Turrialba Volcano mountain bike DESCENT
http://www.costaricaadventures.net/activities/bike-Turr_Volcano.html
39) MOSTLY downhill biking on the Irazu and Turrialba volcanos
http://www.costaricaadventures.net/activities/bike-Irazu_Turrialba_Volcano.html
40) For those who prefer their biking truly bizarre how about this:
http://www.railbike.com.
41) For those who don't like to pedal at all (or even coast downhill), how about a guided motorcycle tour
http://www.wild-rider.com
42) For those who find the w/w rafting tours for beginners, how about a beginner's day class in w/w KAYAKING
http://www.costaricaadventures.net/activities/kayak_lessons.html#Meet_the_kayak
43) For those who find organized group tours too expensive and, well, too organized how about any number of totally free and independent hikes including "the hike of the 3 crosses" in the hills over Escazu (suggested by Chi_trekker, so ask him for details), or I'd suggest a walk along the abandoned jungle train railway near Turrialba (it takes a bit longer to get out there but you're not going to be walking all day anyway and the bus-ride itself is pretty scenic) or you could do as MrP also suggested and just take off anywhere in the mountains surrounding SJ (the local farmers aren't going to mind as long as you're respectful and don't leave a mess). If walking all day long seems to tiring, consider that you don't have to go too far - bring a book or a picnic lunch and a blanket and just stop somewhere along the way with a nice view to relax for an hour or two before heading back.
And finally, for now, there are seasonal activities that may or may not be going on while you're in town. These include:
44) Dia de los Boyeros (2nd Sunday of March in Escazu) with its colorful oxcart parade
45) la Fiesta con Máscaras (March 31st in Barva) with its wild masks and, according to MrP, K*ds pelting each other with animal bladders filled with water
46) San Isidro Labrador's Day (May 15 in Escazu) celebrating the patron saint of farmers with lots of lifestock and praying on display (not really my thing) or
47) la Carrera de San Juan 2 days later (they call it a marathon but its really "just" half but still very gruelling) sign up and run or just watch,
48 ) the 9-day Mangos Festival in Alajuela in mid-July,
49) the nationwide religious pilgrimage to Cartago on August 2nd,
50) San Ramon Day on August 31st with religious processions from neighboring towns and lots of celebrations,
51) CR Independence day on Sept. 15th when K*ds in towns across the country hold home-made lantern parades,
52) Virgen del Pilar's Day on Oct. 12th in Tres Rios with fair, parade and costumes made entirely of corn husks,
53) the Fiesta del Luz (light parade) in San Jose in mid December (actually really more a night time activity than a "day-trip") and
54) the Festival in Zapote during the last week of the year when you can step into the bullring with very brave (or very foolhardy) ticos to taunt the bulls.
etc. etc. I could go on but you get the idea. Some of these are full day trips, but all can be done out of SJ. Many can be done in a half day or even just a few hours. Of those many can be done in various combinations with each other, though from a mongering perspective there is something to be said for a tour that doesn't force you to rise early and leave town for the entire day.
The popular 4-in-1 tour (
http://www.ecoscapetours.com)is often mentioned and that is great for those with a very limited time in CR who don't expect to be back. But be forewarned you spend relatively little time at each attraction and rush from one to the next only to return to SJ well after dark. If you plan to visit often I think it is much better to split it up and do it properly and just save what you didn't get to see for a future trip. There is also an "active" version of the 4-in-1 that combines rafting, ziplines, horseback (actually that would make it only 3-in-1). Here is where I tell about it
http://costaricaticas.com/phpBB2/viewtopic.php?p=61725
Another group of sights that are often combined is Irazu with Cartago and either Lankester and/or Orosi or sometimes Guayabo. Those would be difficult if not impossible to see all in one day if resorting to public bus but can also be done independently (and perhaps more cheaply particularly if done with other mongers or a novia) if you rent a car or hire a driver.
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