My roommate and I decided to take surf lessons at Playa Hermosa, a nearby beach. It is geared towards girls, so it was even better...the waves are perfect for beginners. The sets are slow and they are good for long rides.
Equipment: Towel, bathing suit, and yourself. Might want to bring a rashguard.
I have read Jacó is considered the surfing capital of Costa Rica and Playa Esterillos Este where I took this pic is said to be a surfers' off the beaten path relaxing spot and what about a more relaxing sort of big inner tube instead of a surfing board...? ;-)
Equipment: in Jacó more than one shop rents surfing boats but whatever you feel like doing on the beach, do not forget the sun cream: it was one of my very last days in Costa Rica and I succeeded in getting sun-burnt at 9.30 in the morning...