My favorite place. I haven't been to Costa Rica for over 10 years but considering the pace there, I am sure nothing has changed.
I stayed with friends in San Jose and got to see Costa Rica from a local perspective. I stayed there a total of 10 or so weeks over 3 trips.
One trip, we went to Playa Hermosa and hung out at the El Velero hotel. (This is just south fo where you are going to stay). I would recommend this place. Rates then ran $35 a night but now it looks like they are up to a whopping $50-$75.
This place is rustic and wonderful. Captured the "tico" spirit. In the evenings you can hear the howler monkeys in the trees. The Pacific coast is beautiful.
Looks like you are going to a higher end resort. Although I wouldn't go there, it is probably a good idea for a first trip without having a local to show you around.
I wouldn't recommend getting a rental car and driving anywhere unless you are well insured. Driving laws, road conditions and highway markings are pretty much up in the air depending on where you are.
If you get on some excursions, get away from the group if you can. People are wonderful there and most speak english (it is taught at early grade levels).
There are 2 seasons wet and dry. They can occur in the same day. Mid country temperatures run 80-85 degrees with fair to high humidity. By the beaches it can get up to 90 degrees. On the tops of the volcanos (there are many trips you can go on depending on activity) it can get pretty cool. We played in the snow on top of Volcan Turillalba.
I imagine the resort will have a diluted disney version of Costa Rica. It would be good to break away and go to San Jose for a day trip. (Almost everything in Costa Rica can be a day trip).
Enjoy and make the most of your stay.
