When is your reservation relative to Mardi Gras? The obvious alternative would be, as suggested above, the JW on Copacabana rather that your Sheraton Rio in a somewhat bad location not too far from Ipanema. Alternatively, there are a bunch of new chain hotels out in the suburb of Barra that were built for the Olympics, including a marvelous beachfront Grand Hyatt (but located on the lagoon too). IIRC there's a Sheraton here, but it wouldn't be right on the beach. These places are about an hour from the center of Rio, depending on traffic, and IIRC about a US$50 taxi fare. There's some public transportation, which locals don't consider to be particularly safe but YMMV.