Buy a ticket to watch a game at Gammliavallen. 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.
You'll be spoiled for sightseeing choice when spending time in Umea. Gammliavallen is a great place to begin here, but there are numerous other interesting places to discover. Umeå University, Nolia, and Utopia are not far from the sporting facility and will top off your day nicely.