If your idea of a great vacation means being by the water, make a beeline for Marinheiros Island. Whatever you choose to do here, you're unlikely to get bored of those pretty views.
You'll soon learn why this Rio Grande water-surrounded land mass is popular with travelers. If you're looking for other things to keep you busy, Nautical Museum, Our Lady of Mount Carmel Church, and Municipal Cultural Center are in the area too. They won't take you very long to reach from Marinheiros Island.