Jaco Beach is a great spot to spend Spring Break, there are lots of people coming to surf, party, and have a good time. The best places to stay in Jaco are right near the beach, but you can also get a beachfront hotel room in Playa Hermosa for 1/2 the price. Check out this promotion with Marea Brava Hotel in Playa Hermosa: http://www.mareabravacostarica.com/specials.html
Whether you want do do an ATV tour, surf, or just hang out on the beach, Marea Brava offers entertainment right at the hotel, with dirtbike tours for Costa Rica, surfing tours in Playa Hermosa, and more.
Of course if you like boating, then Los Suenos Marina in Herradura is just down the road. Los Suenos offers a full marina, golfing, restaurants and a very high-end experience. If you travel here and stay at Los Suenos, be sure to stop at Jaco and try some sushi at Arrigato or one of the other fine restaurants.