I agree that he should make it an overnight and normally I wouldn't recommend to anyone to take the 1 day tour from SJ EXCEPT for one important thing - you don't HAVE to return with the rest of the group that SAME evening. They WILL allow you to bring your luggage (which in this case need not be more than a few extra items stuffed into a large day pack or overnight bag) and then return with them on a later date (or even return to SJ or head on to somewhere else on your own). Of course you could also do it with Interbus ($39 ea way or 5% less if booked thru
http://www.shuttlebuscr.com) or Grayline ($35 each way), but those shuttles don't stop in Grecia and Sarchi. And, if you went with the day tour, for only about $10-40 more than what you'd pay for those r/t shuttle tickets (depending on which tour and which hot spring) you'd get admission to a hot spring that you'd probably be paying to visit anyway (Tabacon's cheapest package is now $70/pp and Baldi is $42/pp, though discounts may be available), lunch and dinner (although 1 of those is already included with those hot spring prices) and usually some stopover time at those sightseeing spots along the way.
http://www.grayline.com/Grayline/destinations/latinamerica/costarica.go?mode=activityDetail&apid=1961&sid=570 $109/pp no mention of stops along the way but does specify Tabacon
http://www.cupotico.com/coupons/activity/activity29/activity29.html $120/pp no mention of Grecia or Sarchi stops but does mention Zarcero and Tabacon
http://www.costaricatours.com/oneday/oneday05.html $130/pp no intermediate stops but a mid-morning departure, Tabacon AND a canopy zip-line tour
Based on pricing and descriptions, these last 3 are probably all the same tour (w/2 hot springs to choose from depending on how much you want to spend) just being advertised by different tour agencies:
http://www.costaricantrails.com/tours/san-jose/arenal-volcano-by-night2.html $89/pp for Baldi and $114/pp for Tabacon with stop in Sarchi along the way there
http://www.viator.com/tours/San-Jose/Arenal-Volcano-and-Hot-Springs-Day-Trip-from-San-Jose/d793-4100AVHS $89/pp no specific mention of stops or springs but a "souvenir stop" (probably Sarchi) was mentioned in one of the reviews, as was both Baldi and Tabacon.
http://www.rainforesttours.com/sjoneday/arenal_tabacon.htm $114/pp Tabacon & Sarchi specified
Of course, like I said before, you'd have to pay for a hotel room in addition to the day tour fee but that still should come out much less than the $250 PLUS seperate hot spring admissions.
La Fortuna is a nice small town (particular for those visiting w/o their own car) and Volcan Arenal is best seen at night, HOWEVER the town is NOT the best location for night time volcano viewing since the lava has been flowing on the other side of the mountain and the rainy season is not the best time for lava flow viewing anyway. OTOH, Arenal should STILL be seen as an overnight trip if for no other reason that there are so many OTHER things worth seeing and doing there and because it is such a long trip from SJ.
As for how much time you need in Grecia and Sarchi. IMHO, unless you're also stopping there for lunch, Grecia need not take more than a half hour if that since the metal church is the main thing to see and the town probably comes too soon along the way for lunch and Sarchi could take longer depending on how much of a shopaholic you are but shouldn't require more than a hour or maybe 2 if you include lunch. I'd imagine the shuttles if they stop in those towns at all, it would only be VERY briefly but even with the day tour the stops are probably not much longer. Also, again IMHO, both of those places can easily visited independently and you don't really need a guide to appreciate either of them.
Personally, I wouldn't do this with either SL, a day tour or use any of the shuttle companies. Taking public buses to/from Fortuna is pretty straightforward and well under $10 each way. And even if you want to do stops in Grecia or Sarchi you could easily do that by bus. Buses to Grecia leave every half hour, take an hour to get there and cost only a buck or two. Buses from there Naranjo stopping in Sarchi leave every 25 minutes and take another 40 minutes to get to Sarchi for even less. And, if you plan your departure time from Sarchi correctly, with a quick change of buses in Naranjo you can be in Fortuna 3 hours after that, for a total transit time not much longer than what it would take you to drive that route yourself (e.g. SJ>Grecia 8AM-9AM, 40 minutes in Grecia, Grecia>Sarchi 9:45-10:25, 2hrs in Sarchi including lunch, Sarchi>Fortuna 12:30-3:30PM or you could hit those towns on the way back the next day)
Yet one more thing I often recommend people consider is to combine travel to/from Arenal with a rafting tour that was probably on your agenda anyway. If you go with
https://www.exploradoresoutdoors.com/, you can get picked up in one location and dropped in another for no extra charge over the regular $99/pp you'd pay for a Pacuare trip anyway. You could do this heading out instead of the day tour I suggested earlier (and then take either the shuttle or bus back, hopping off the bus in those towns if you want to). OR you could travel TO Arenal by day tour, shuttle or bus and RETURN to SJ using the rafting tour (in the case of the day tour that would mean giving up your "free" return ticket, thus negating some of the cost advantage I mentioned earlier).