I’m thinking about going for a day trip in mid-October…most likely from CDG and back in the same day.
The plan (could be)
Arrive into Brussels at 9am
Depart Brussels at 836pm (arrive CDG at 9:50pm)
So, this gives me about 11.5 hours…
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The quick train connection between the 2 cities is really great. I love Brussels but I don’t go there for tourist stuff, I go there to see friends so I’d be quite unable to tell you what to see. There is the Palais Royal, the Grand Place and the Mankenpiss… that’s a start. You can also have the best fries ever, and some great waffles and chocolate. The beer isn’t bad either 
Where on the waffles and fries…unfortunately…I’ve given up drinking beer.
Sprouts?
The Manneken Pis is pretty funny.
I didn’t think the fries in Brussels were any better than the ones in Amsterdam or Paris, but the beer was out of this world. And I’m NOT a beer drinker.
If you’ve ever watched Cathouse on HBO, the Petite Rue des Bouchers has a vibe sort of like the line-up at the bunny ranch, only with moules-frites instead of… well, you know.
A lot of the chocolate you’ll find in and around the Grand Place is available in the US. Steve and I hauled home boxes from Leonidas and Neuhaus only to discover that both are here in Portland, and cheaper than they are in Brussels. Seek out the ones that aren’t available at home.
I like Brussels. As Guillaume mentioned, the Grand Place is remarkable, and only about 100 meters from the central train station. You could easily spend a day hanging out in that immediate vicinity, which includes the Mannekin Pis and the Rue des Bouchers that Melissa mentioned.
Another option would be to take a day trip within a day trip and go out to Bruges (Brugge), which is a really scenic place to spend a few hours. You can book a connection from the airport via Brussels North at http://www.b-rail.be/int/E/services/tickets/index.php.
According to the other thread it’s already a day trip from Paris so I guess a day trip out of a day trip won’t work out 
Michael, Maison Antoine on Place Jourdan has a huge reputation but I’ve never been there. I can recommend Friterie Martin on Place St Josse or the Friterie on Place Fagey.
Brussels to Brugge can be a 2-train trip (change in Ghent I think), even if one, it adds another 80 minutes plus the walk to the centre is about a mile so your 11 hours all of a sudden becomes closer to 7. Good chocolate that you can’t get elsewhere in EU is Wittamer, on Place des Grands Sablon, also another chocolatier with an Italian surname that esacpes me (Marcolini?) or is at least hazy in same square (both pricey), but I also like the cheapie Leonidas and usually load up on that for gifts back home for those who do not have passports. Skip Mannekin Pis and hunt out the frites and waffle stands, however near Mannekin Pis last time I was there an escargot vendor was hanging out - nice fast food and definitely not Mickey D’s.
Marcolini–yep.
I love tacky souvenir shops, and there are some great ones around the Grand Place. One of the popular souvenir designs has the Mannekin Pis peeing out the word “Brussels”. Those were popular with the kids.
Here’s that escargot stand from our '07 visit…
Bought my TGV ticket today…looks like I’ll be there for sure. Thursday October 15th.
Will be in Brussels for three days starting this Sat…
Interested in dining suggestions, especially any place that serves really good mussels and frites. Will be spending at least one day in Beersel, as well.
Ahillya - I think most people on this board are looking for personal experiences that actually answer the posters questions instead of just a cut and paste out off a website.
George