Purchase a ticket to watch a game at Stadion am Volkspark. If there's nothing scheduled while you're there, you might like to check if there's a guided tour of the grounds.
This Flensburg sporting facility is an interesting place to kick off your day of sightseeing. There's loads to see and do close by, including Flensburg Harbor, Gluecksburg Castle, and Solitude Beach. Wander around at your leisure at Stadion am Volkspark before making tracks to these other fine sights.