In addition to buying a Eurail pass (you don't have to do the whole trip by rail), there are the bike share systems in the downtown areas of Paris and Brussels. (Brussels' bike share system was implemented just a few wks. after the international Velocity bike conference in 2009.)

Pic of Paris bike share system here:
http://thirdwavecyclingblog.wordpres...oid-some-pain/

Both cities have subway systems also. Save yourself the car agony, at least when travelling within those big cities.

I've been to Bruges awhile ago for a few days and my partner was there just last year for a few days in Belgium. With its medieval canal system and other narrow roads in downtown area, might be more fun to leisurely cycle along anyway.