A few random ideas from several years ago. I haven't been recently.

Train to Leuven - Stella Brewery
Train to Brugge - see the Cathedral
1958 World Fair grounds - Google "Atomium" - interesting structure
Cantillon brewery
Musical Instruments museum
Comic Strip museum

Typical layover stuff includes:
-Grand Palace, Delerium, Morte Subite, Celtica, and a few others the crew will know.
-For a first time visitor, go see Manneken Pis

Check out Rick Steves travel guides and google "Belgium walking tour" You can download a walking tour for almost any city to your iPod or phone. Safety tip - Only use one earbud!