Purchase tickets to see a game at Gremio Arena. If there's nothing scheduled while you're in town, you might like to check if they offer a guided tour of the grounds.
You'll find this attraction in Farrapos. Pay the entrance fee and then explore at your leisure. If you still have time after Gremio Arena, make tracks to nearby sights like Moinhos de Vento Park, Porto Alegre Public Market and Rio Grande do Sul Museum of Art.