Buy tickets to watch a game at Stade Sylvio Cator. If there's nothing happening while you're in the area, you might like to check if there's a guided tour of the grounds.
You're spoiled for sightseeing choice when spending time in Port-au-Prince. Stade Sylvio Cator is a good place to start here, but there are plenty of other interesting places to check out. Champs de Mars Square, Port-au-Prince Cathedral, and National Palace are not far from the sporting facility and will top off your day nicely.