On my way to Prague from Rome, I was very lucky to get a Brussels Airlines flight with a 7 hour layover in Brussels. This enabled me to walk out of the airport for a couple of hours and see the sights and savor the sweetness of the city center. Rode Bus 272 for just 3 euros and it shot me straight to Brussels Nord where the historical center of the city is just a 20 minute leisurely walk away.
No matter how short the time given, there's always a place waiting to be explored!
Steve