Do *not* visit the Atomium. It's closed at the moment, and even if it weren't, it's utterly crap.
If your meeting is in Waterloo then you could take a look at the battleground and the museum next to it, also just walk up Waterloo High Street, it's rather nice. Take in a café, just hang out.
On the other hand, if you do have time to kill and if you can get into town, then I suggest you head straight for the centre, start in the Grand Place. Get yourself a beer at the Roi d'Espagne in the corner, then take a look at the wonderful Town Hall. After that, walk to the Galeries St. Hubert, then down the Rue des Bouchers and see if you can find a fantastic little pub called Toone - it's not easy to spot as it's down an alleyway, but it's wonderful. It's an old puppet theatre and it does all the great Belgian beers. You could also try the Mort Subite at the other side of the Galeries, but it tends to be very crowded and noisy.
Avoiding the touristy shite like the Manneken Pis and stuff like that - and above all avoiding the food down the Rue des Bouchers which is purely for Yank tourists - head up the hill towards the Galerie des Beaux Arts and the Olde England department store - a fantastic art nouveau building which now houses the Museum of Musical Instruments. At the Place Royale turn right, past the gallery, and walk down Rue Royale until you get to the Eglise de Notre Dame du Sablon. Look at the Petit Sablon park on the left, then turn right and wander around the Grand Sablon, a beautiful mediaeval square with lots of great pubs, restaurants, and chocolate shops. Afterwards you can walk down the Rue de Rollebeek and head back into the middle of town - alternatively walk up to the Palais de Justice, admire the massive building, and also take in the incredible view over Brussels. You could then walk up the Avenue Louise, the Mayfair of Brussels, maybe grab something to eat.
Shit I wish I was there to show you around mate!
I recommend this book
if you can get your hands on it. He's a top geezer and has some great little tips and walking suggestions. May seem like overkill for one afternoon but I'm sure you'll get plenty of use out of it, you should look on this as a scouting mission to take your missus over there for the weekend one of these days! Anyway, you'll have to come & visit us when we get back there.