Get your tickets and join the fans at a game at Fratton Park. If there's nothing happening while you're in the area, you might like to check if they offer a guided tour of the grounds.
You're spoiled for sightseeing choice in Southsea. Fratton Park is a good place to begin here, but there are loads of other worthwhile sights to discover. Gunwharf Quays, Spinnaker Tower, and HMS Victory are near the sporting facility and and will cap off your day nicely.