Eat some paico (type of ice cream)! Costs less than $2 and you can customize your scoops :D We highly recommend you go paddleboarding in la Laguna de Condado; rent the boards right by the bridge. Please do not go swimming in the beach behind the hotel "La Concha", as it is not a swimming beach and thus unsafe for tourists.
Get yourself a piragua (Puerto Rican shaved ice dessert). Z's opinion: blue raspberry is the best flavor, hands down. You will find this near the beach or while exploring San Juan.
You can go hiking, swimming, camping, have a bbq or even have a picnic! We recommend getting there very early as there is limited parking. Bring your own food and drinks, as you will not find any stores inside the rainforest. If you decide to go hiking, try the Tres Picachos, but please notify a ranger before you start your ascent.