Buy a ticket to watch a game at Stade Chahid Hamlaoui. If there's nothing scheduled while you're there, you might like to check if they offer a tour of the grounds.
You're spoiled for sightseeing choice when visiting Constantine. Stade Chahid Hamlaoui is an ideal place to start here, but there are plenty of other interesting places to visit. Emir Abdel Kader Mosque, Palace of Ahmed Bey, and Cirta Museum are near the sporting facility and and will cap off your day nicely.