I just got back about a month ago from Tamarindo and had a great trip. Pretty much did the same stuff as the poster above, mostly relaxing on the beach but a few days of surfing. I also went to Rincon and went rafting (its currently the dry season, so its the waters really slow and boring to raft), and zip lining. I can confirm that its about an hour or so from Tamarindo.
My younger brother (16) had no problem getting drinks anytime he wanted them, they'll serve basically anyone. The estuaries are pretty cool, the bars are fun, and it sounds like you'll have a great time there.
Pm me if you want some more information.
Manuel Antonio and Playa Grande were both pretty sick. Definitely go check out the Manuel Antonio public beach and the national park. If you get to go to the Potrero Bay area (pretty close to Playa Grande), head over to Playa Flamingo and go to the Mar y Sol restaurant around sunset. They have a bar/lounge upstairs that overlooks a cliff and it was one of the most amazing sunsets I've ever seen. The food was also really good.
this is all so sick to hear. im going to costa rica for 3 weeks in july for a summer biology program