If you're a fan of live theater, book yourself a ticket for a play or other stage show at Camara Tulio Piva Theater. Check online to see what's still available.
To explore this arts and culture attraction, find your way to Cidade Baixa. There are loads of other things to see in this area, such as Beira-Rio Stadium, Porto Alegre Public Market, and Moinhos de Vento Park.