One of my absolute favorite things to do in Leuven is to go to the produce market in the center, right next to the Town Hall. You can find fresh fruit and vegg, mostly Belgian grown but also exotic fruit. The smell of strawberries draws you to it from far away. You cannot pass by there and not have a look and buy something. For the tourists traveling on a budget it's the perfect place to buy fresh, healthy food for little money.
Other vendors sell olives of all kinds, cheeses, chocolate and other sweets. You can find food and coffee booths. I love it. Many times we go just so I can walk around the market and the prices are much cheaper than all other markets I have been to in Belgium. My guess is because the market is there everyday. It's allot like a supermarket in the open air.