I’d suggest Bar Lagoa for a very good meal and great draft beer. There’s also a Porcão on Aterro do Flamengo where you have a great view of the Sugar Loaf.
For a great sandwich late at night try Cervantes in Copacabana or Barra. Very good after a night of drinking and dancing (or after a whole day at the beach).
If you by any chance have time for a little side trip, go to Petropolis (about one hour by car from Rio) and try Alcobaça. Wonderful place, lovely owners and a meal to die for.