Metrô Largo do Machado or Catete
First things first, if its a sweltering hot day and you want a bit of beach action to cool down, then Flamengo's little offering may not be the best port of call. The sand is clean and the bay inviting, but being on Guanabara Bay the waters are sadly an official no-go for cleanliness, and though a few hardy souls do venture in, one look at the brownish-green waves lolloping onto the shore should probably be enough to put you off anyway. However, running from the outcrop of land on which Porcão Rio's sits, all the way up to the Marina da Glória is a sweep of sand that is awash with activity from sun up to sun down, whether it be frescoball fanatics ('bat and ball' to the casual observer but sometimes impressively competitive), volleyball matches (either with conventional use of hands or the head and foot combo known as futvolei) or some good old fashioned castle building, and on weekends and holidays it gets as packed as its more fashionable counterparts down the coast.
And therein lies its beauty. Families gather around guitars and have a pre-barbecue singalong, kids tear around safe in the knowledge that the park is sufficient barrier between the beach and the road (which shuts on Sundays anyway), and the whole experience feels like a genuine slice of carioca life for the majority of the city's inhabitants. The rocks to the southern end are a fine vantage point for fishing, and the aeroplanes zipping in and out of Santos Dumont airport add further to the feel of a real urban beach, reinforced by the vision of Niteroi across the bay. On a clear day, the mountains of Friburgo still further beyond, and Sugarloaf much closer, make for stunning views too.