Keep an eye out at Grand Market Square — you're sure to come across some treasure or funny trinket among the stalls here. After you've admired the wares, grab something to eat from one of the nearby food vendors.
You're spoiled for sightseeing choice when visiting Leuven. Grand Market Square is an ideal place to start here, but there are lots of other worthy spots to check out. KU Leuven, UZ Leuven Gasthuisberg Campus, and Leuven Town Hall are close to the community marketplace and will fill in your day nicely.