Hi Terry, you don't mention what time of year you're traveling, or what your budget is. We stayed 2 nights in Brussels, saw the tourist sights in Brussels the first day, then did Bruges as a day trip from Brussels. You could easily do the reverse, if you want to stay in the more charming city of Bruges. Brussels to Paris is a quick trip on the TGV, which will require advance reservations.
If you are going in Spring time, you should visit Keukenhof Gardens, which is between Brussels and Amsterdam. We stayed 2 or 3 nights in the Hague (Den Haag), which has a lovely wide (though touristy) beach, Scheveningen, with great tram service from downtown. We spent one day at the beach, one day at Keukenhof Gardens (tulip capital of the world)then did Amsterdam as a day trip from The Hague. The hotels are less expensive than Amsterdam, and we appreciated the nice beach. One full day in Amsterdam was enough to take in the Van Gogh museum, Ann Frank's house, and the Heineken museum tour. You will probably have a line at all 3, so check to see if you can buy tickets on line in advance.
Yes, there is no doubt, you MUST do Paris!! I could write a book about why you should visit Paris, but check out my blog, I have recently posted 3 great day trips from Paris, plus the Normandy D-Day beaches.
Paris is even beautiful in the rain. Here's a link to a short video I shot last December http://enchantedtraveler.wordpress.com/2008/11/30/paris-on-sunday/
Beware though, the more you research Paris, you may decide to spend all your time in France. What is your budget for a hotel in Paris? I have stayed in all prices ranges, including 2 star and been happy. Just know that rooms are small. Enjoy, and bon voyage.