You should be able to get a good look at the area around Grand Place with that time available. Let's back time this. Buses leave virtually every hour from right outside the Brussels Zuid/Midi station, which is only a short distance from Grand Place. With a flight leaving at 18:20, you'll probably want to be there around 16:20, which means catching a bus around 15:20 for the hour-long trip. If you're flight is on time, you should be able to take the train into downtown Brussels by 11 am. That should give you at least 3 1/2 hours clear. Check http://tinyurl.com/28f6ko for the current bus schedule to Charleroi, and make sure you double-check just before you leave. BTW, don't fall for the taxi-drivers' scam that the bus service is delayed because of mechanical problems.