Buy tickets and join the fans at a game at Mansfield Oval. If there's nothing happening while you're in the area, you might like to check if they offer a tour of the grounds.
You'll have no end of sightseeing choices when spending time in Mansfield. Mansfield Oval is a great place to start here, but there are lots of other fine attractions to experience. Goulburn Valley High Country Rail Trail, Delatite Winery, and Mansfield Zoo are not far from the sporting facility and will top off your day nicely.