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're spoiled for sightseeing choice when spending time in Farrapos. Gremio Arena is an ideal place to start here, but there are plenty of other fine spots to enjoy. Porto Alegre Public Market, Moinhos de Vento Park, and Sao Pedro Theater are not far from the sporting facility and will round out your day nicely.