There is a new water park in San German (west part of the island).
In Ponce (south side) you can visit the "Parque Iindijena" (Indian museum), "Plaza de Ponce" where you can visit the oldest firehouse. Take a ferry in 'La Guancha" and visit "Caja de Muerto" (coffin island), visit "La Cruz del Vijia" (watch tower) where you can see the city and listen to the history, visit "El Castillo Seralles" and walk thru the gardens. Most of this historical sites in Ponce are free on low cost and can be done in one day.