If your idea of a great vacation means being by the water, make a beeline for Ilha de Itacuruçá. No matter what you choose to do here, you're unlikely to tire of those pretty views.
Ilha de Itacuruçá is a water-surrounded land mass in Mangaratiba. If you're looking for other places to visit in and around the area, you can't go past Muriqui Beach.