In 24 hours you could stay at either Copacabana or Ipanema beach. A friend of mine did a day-tour of Rio in which they went up the Corcovado, Pão de Açucar, visited the Sambodromo and the Maracanã museums and went to the beach. This is, imho, too much of a good thing. I would skip one of Corcovado and Pão de Açucar (probably Pão de Açucar, because if the weather is cloudy, at least you'll see the Cristo on the Corcovado), and not bother with the Sambodromo either. If you aren't into football skip the Maracanã too. You could instead spend some time wandering up and down Ipanema and Copacabana beach, or even visit Centro to see the horrendous architecture that is the cathedral, the Selaron stairs, and some of the prettier colonial churches. You could go into a museum. Regardless, there is more to do in Rio than you will have time for in 24 hours, and what should be the highlights depends on what you want. Hell, if you want to party, head to Lapa at night, and sleep in the next day ;)