Porto das Dunas
A huge water park with slides and other wet interactive fun, Beach Park . Stiffly priced at R$ 120 for a day, but with substantial discounts for subsequent days. Out of season they typically close once or twice a week, although rarely on weekends. The beach right outside the park is very nice, and although you pay nothing to walk in the sand, food and drinks are 2-3 times city prices.
There are several pricey hotels in the Beach Park complex.