If you have a long layover and want to visit Rio de Janeiro:
Be aware that you have to have a visa in order to clear customs. For US visitors, a tourist visa is $100, and cannot be acquired at the airport, so plan ahead. If you are traveling elsewhere in Brazil with a long layover at Galeao Airport, this may be your chance to visit one of Rio's beautiful beaches, see the famous Christ the Redeemer statue, or other sites. The airport shuttle service offers inexpensive and fairly quick transit to the beaches and other major areas of the city, and taxis are quite affordable.
The beaches of Rio offer a host of excellent food establishments. Also, Rio is home to many world class museums, churches and cultural facilities.