Ryanair - careful though. Ryanair flies into both Charles De Gaulle and Beauvais. Beauvais is a full hour and a half from city center.
Have you thought about just renting a car? A one-way from Paris to Brussels is about $500 for a week (i've seen as low as $260 for a compact on expedia) with the one-way fee included. Drive time is only about 7 hours including connecting through Amsterdam for the dutch show. It would be cheaper than rail or flight for two or more people, roughly the same amount of time as rail and less time than flight (when you add 2-3 hours to each flight for travel/getting there early etc) .
Also, it gives you the chance to stop off in some truly amazing places:
Bruges: Everyone must see Bruges at least once in a lifetime.
Dunkirk: Sight of the famous world war 2 British evacuation
Baarle-Hertog: The worlds weirdest border! It winds around in knots and has parts of Belgium fully encircled by Holland and vice versa.
Ardennes: See the place where American soldiers abandoned a whole fleet of german cars in the forest - and they are still there today, protected by the Belgian government as time consumes them
..and more.... Live a little! Drive it :-)