My wife and I did that exact trip this time last year. We took a little over three weeks and ate and drank our way through German, Belgium, and France (we didn't even go to Paris, just stayed in wine country).
In Germany, we went to:
Munich: Ayinger, Kloster Adechs, different biergartens around the ctiy,
Ettal: Kloster Ettal (about an hour, maybe an hour and half south of Munich)
Bamberg: Home of smoked beer. 7 fantastic breweries, collect all 7

Aachen: Not a lot of beer stuff, but a nice break before moving to Belgium. Visit the Bad
here, trust me.
In Belgium:
Bruges: All the pubs, all the food, all the beer. We just walked around eating and drinking
all day
Westvleteren: St Sixtus, De Struise, De Dolle...we looped from Bruges and made it a day
trip
Brussels: IMO, it's just another big dirty city just like we have here in the US. We drove
through it only to stop at Cantillon on our way to France.
In France we skipped Paris and made a big loop around the country like this:
Champagne-->Alsace--->Burgundy--->Bordeaux--->Provence
We did the train thing all through Germany, but then rented a car in Aachen to get us around Belgium and France. A lot of the places we went in those countries were not conducive to public transportation.