Zip lining - 100% recommend doing this. It will be the best $50 you've spent in a long time.
If you're looking for a serene beach with all sorts of amenities and activities, check into Playa Flamingo. You can fish, snorkel, dive and hang out on the beach for hours on end.
If you're looking to escape the mountains and head to the beach, try Tamarindo
Puerto Viejo is a small town on the Caribbean coast. This town is tourist friendly, full of the blend of Costa Rican and Jamaican culture for which the Caribbean is famous. Delicious coconut based food, tropical drinks, and reggae music. Bring a rain coat for safe measure.
If you want to check out the Atlantic side of CR, check out the world famous sloth sanctuary!
Just a short trip from San Jose is Volcán Poás - a nice walk and close up view of the crater!
If you're looking for an adventure you can do in a day; zip lining, horse riding, white water rafting, this place is for you. Located a few miles north of the city of LIberia, near Rincon de la Vieja volcano.