Buy a ticket to watch a game at Sugathadasa Stadium. If there's nothing scheduled while you're in the area, you might like to check if the stadium offers a guided tour of the facilities.
Pay the entry fee and then take your time to really explore this Colombo sporting facility. If you're looking for other fun things to see and do in this area, Galle Face Beach, Galle Face Green and Ballys Casino aren't too far from Sugathadasa Stadium.