For Brussels, Cantillon is a must see, one of my favourite breweries in the world, but in a bit of a dodgy part of town, locals may not know it, or may know it as the Gueuze Museum (Musee de la Gueuze). Find it and set aside some time, taste all the things.
Moder Lambic is an awesome craft beer pub with Cantillon on Hand Pump and other local beers available. A little pricy by local standards, but cheap as frites by Aus standards.
I loved Ghent, it was a little less touristy than Brugge, and with a main street full of awesome castles and cathedrals and town halls... They also have a brewery named Gruut, which makes tasty belgian style Tripels and Quads etc. but without any hops at all. Have the Charcuterie Board!
http://www.gruut.be/eng-introduction.html
In Brugges, definitely do the De Halve Mann tour, they've got a great stable of beer.
And while in town, check out Cafe Rose Red, a great little pub with a slammin bottle list, sometimes including the elusive Westy 12
http://www.tripadvisor.com.au/Attraction_Review-g188671-d4322161-Reviews-Cafe_Rose_Red-Bruges_West_Flanders_Province.html
But if you're going to drink Westvleteren, go to the source. You may not be able to get into the monastery, but you can get close, and the cafe across the road (
http://www.beeradvocate.com/beer/profile/2169/ ) has a great food and an atmosphere befitting the almost mythical status of the beer. Be warned, it took us three trains and two busses to get there. So Worth it. It may or may not be the best beer in the world, but the adventure to get there, the exceptional beers and the vibe of it all was definitely up there with the best beer experiences in the world.
We stayed in Poperinge, a cool little town with a long history in the hop industry. they have a hop museum that was worth a look, a little school field trippy, but interesting.
Het Anker Brewery (who make the Gouden Carolus beers) is in Mechelen, just south of Antwerp.
http://www.hetanker.be/en
The brewery and the town are beautiful, I would highly recommend a visit. The brewery have a bed and breakfast, the hotel style rooms are nice but simple, but the package we got included breakfast, a 4 course dinner with beers, take away bottles and a brewery tour. and once the bar closes, the common library area in the accomodation has a vending machine full of corked and caged bottles which would be upward of $25 here, for around 5 euro each.
Climb the local cathedral bell tower the day before, because your stomach might not handle it the morning after.
