Circus is a wise sage.
Monteverde is a must see from a purely naturalist viewpoint and
http://www.monteverdeinfo.com is perhaps the best site out there on the area with tons of valuable info. Also as Circus said 2 nights is a suggested minimum and a poncho is a must this time of year (rubber boots you can pull on over your shoes wouldn't be a bad idea either). Another possibility that I haven't tried yet but have always wanted to is to take a horseback/boat/jeep trip from Monteverde to Arenal and return to SJ from there after spending a night. Just be sure to go with a reputable outfit (PM me if interested - I can dig up a name).
As for poontang, I'd bet that is your major reason for visiting CR, but I wouldn't recommend trying to find a MULTI-DAY companion in so short a time for your first trip. What seems pleasant for ST or even Todo le Noche can wear very thin over a couple of days and in Monteverde there is no easy way to send her packing. You should decide whether you want to devote yourself to non-stop pussy-pounding on your trip or whether you can afford to spend a few days without it.
IMHO, while I recommend Monteverde for a visit, if not now at some point in your CR travels, but if you want to make the trip with a female "guide" I think either the beach or Tabacon would make a much better destination.