I don't know if you could pick a giant city more identified with the beach than Rio de Janeiro - between bikinis, Carnival and supermodels; it seems that Rio is constantly being shown with the beach and someone tan and having fun.
When you hear about beaches in Rio de Janeiro, the two most mentioned are Copacabana and Ipanema. These are both nice spots, but Leblon Beach is my pick for the beach to relax upon while in Rio. Going from east to west on the main stretch of beach in Rio, Leblon beach is a the western end of the three.
Compared to Ipanema and Copacabana, Leblon is a bit more narrow, has a couple of nice surfing spots, is not crowded, away from the mass of roving beach vendors and fronts a residential neighborhood so it generally seems quieter.
The sand on the beach has a nice texture and is a khaki color - the beach is clean and there are plenty of opportunities to break a sweat with beach volleyball and beach soccer courts set up. One thing, if you want to use the beach shower, make sure you find out who 'owns' it and pay them a dollar - seriously, the showers are not public in the U.S. way and somebody will turn on the water so the shower will work, but only after you pay them! Also, at about 50 yard intervals on the cement boardwalk that fronts the beach, there are kiosks that have everything from water, to beer, to coconuts (seriously) to sandwiches.
Leblon is not the young, hip, touristy beach in Rio. Rather it tends to be geared towards locals, families or visitors who just want to hang out on the beach. You still can get anything you want while also grabbing sun or people watching because of the walking beach vendors, but it does not have the steady stream of them like further east.
The water is clean, similar to ocean at Los Angeles - a blue grey color. It can be crisp, much more chilled than I was anticipating. I have been to Leblon in both winter and summer, with the winter water being much colder. Do be aware of the current when the surf is up a bit - even though the waves are further out, the rip current can be strong - I would not be too comfortable with younger kids in the water.
Having visited during both seasons, the summer months tend to be more humid and rainy - of course the winter is not as hot, but not as much as I was expecting. All and all, the year round weather makes going to the beach possible, with each season a pleasant experience. Rest assured, if it is sunny, you will be warm and you will get good tan.
The touristy things in Rio are all very condensed - the other beaches, Sugarloaf, the Cristo statue - while everything is close, the traffic can be brutal because the physical layout of Rio is in and among mountains - tough to jam 8 million people into a small place!
Even though Leblon is a residential area, there are a few hotels in the neighborhood and many, many more as you make your way east through Ipanema and into Copacabana. I stay at the Promenade Palladium Hotel, which is just a few blocks inland from the beach. It has simple, clean rooms, a nice pool/sun/outdoor shower area, it is not too big and the staff all speaks English. I have heard that on the east side of Ipanema, there are also some small, good hotels.
A negative about Leblon Beach, and for that matter Ipanema and Copacabana, is the muli-lane road (basically a freeway) that runs along those beaches and is a thoroughfare to the big cities that border Rio's east side. They turn the lanes to head into Rio in the morning and out of Rio in the afternoon - it is not quiet and with all the cars, you can smell the exhuast.
Rio de Janeiro is known as the most expensive city in Brazil - especially along the beaches; they are not shy about making money from the tourists (I hear more German/French, than English spoken when I am there). With the current exchange rate being so poor for the Dollar - Rio is even more expensive - so much so, I would not take a pure vacation there because of the costs associated with the exchange rate.
If you are down south on business and want to spend a mellow day on the beach, then Leblon Beach is the place to go!