Buy a ticket and join the crowds at a game at Theodoros Vardinogiannis Stadium. If there's nothing on while you're in the area, you might like to check if there's a guided tour of the facilities.
Theodoros Vardinogiannis Stadium has an admission fee and is a well-known attraction in Heraklion. There are plenty of other options close by, so why not hang around a while longer? Heraklion Archaeological Museum, Palace of Knossos and Natural History Museum of Crete are also a hit among many visitors.