Buy a ticket and join the crowds at a game at Estadio Municipal de Aveiro. If there's nothing on while you're in the area, you might like to check if the stadium offers a tour of the grounds.
You'll find this sporting facility in Aveiro. Once you've done everything you can at Estadio Municipal de Aveiro, have a look at nearby Ria de Aveiro, Aveiro Salt Pans, and Aveiro Cathedral.