We went to Costa Rica a few years ago, loved it! So many animals everywhere, it was magical. I would say Manuel Antonio for the beaches and wildlife, we stayed at Hotel Mariposa and it was beautiful. We saw such diversity of wildlife it was beyond cool. We also enjoyed Arenal, the whole area is magical, misty and other worldly. Went to Guanacaste and while we were there it was very hot and dried up, but if you are going right after the rainy season might be better. A lot of people were there to dive. To me, it was way too touristy though and if I were short on time I wouldn't bother. Monte Verde was also cool, if you're an adrenaline junkie you have to do the zip line, INSANE! They also have a medicinal plant walk and wild orchids everywhere.
You are taking my dream trip! Next place we're going is Peru and Bolivia to see the ruins but I've been holding off until next year for a big trip away from the kids, just in case...