Purchase a ticket and join the fans at a game at Stadion im. Alfreda Smoczyka. If there's nothing scheduled while you're in the area, you might like to check if the stadium offers a tour of the facilities.
When you've finished up at this sporting facility in Leszno, why not check out St. John’s Church, Centrum Nurkowe Octopus, and The former Synagogue of Leszno? They're not very far from Stadion im. Alfreda Smoczyka and also a hit with many tourists.