If you want a day by the beach where you can walk into the sea and not battle against breakers get out east to Cabo Frio.
On the Praia do Forte you can walk straight out into the sea up to shoulder depth, look down, and still be able to see your feet, The water is that clear! And because of the configuration of the bay it is pretty much always calm.
At Carnaval many Cariocas head out here - plus Paulistas (from São Paulo) and Mineiros (mostly from Belo Horizonte) to get away from the mayhem and tourists in the cities.
Make an early start and head over the Rio-Niteroi bridge and keep going east along the motorway. It shouldn't be more tha a couple of hours drive if you go mid-week but well worth the trouble. Google "cabo frio rj" where you will find lots of sites and pictures.
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