Was just there in February (brrrr!).
First thing's first, pick up the Good Beer Guide Belgium (might be kinda hard to find, it's published in Britain) – everything beer-related that we did and enjoyed, we found in that book (his counter-recommendations are also solid; do not order the Pink Killer!!).
Cantillion is in Brussels, and it's pretty awesome... you wander through the bowels of a working brewery in a way you never, ever, evar could Stateside – like, wort drips on your head as you walk under the pipe from the lauter tun to the boil kettle, you climb a funky attic latter to gawk into the coolship close enough to spit into it, etc. They promote themselves as a brewery and beer museum, so you kinda get the sense they intentionally resist modernizing anything just to keep it authentic, but it's still pretty awesome. They've also got a little bar on-site where you can rare, delicious sours for like $8 (at least, long as the exchange rate remains favorable), even if you don't pay for the tour, ya know, because you did it two days ago, but just wanted to come back for more beers, dunno who'd do that...
If you also at least somewhat like sours, Moedor Lambic, also in Brussels, is a beer bar worth a stop – they've got a couple straight, unblended Cantillion lambics plus another half-dozen or so sours on draft, which is pretty rare (they're mainly distributed in bottle form).
There's a brewery Bruges, don't know if the tour is any good, but the beers were kinda meh. However, there's a gem of a beer bar hidden down a tiny alley called Garre, might be a bit dark and medieval when the sun's shining, but it was a great place to get out of the blustery, drippy February elements.
Otherwise, the public transit is pretty good, we were able to get to several other sour breweries by bus/train without too much effort, but nothing I'd say you have to see (Oud Beersel is small and funky; Lindeman's is an impressive modern operation which still finds time for a couple decent sours among the over-sweetened fruit cocktails even the tour guides call "commercial"). And, I can't really comment on the abbeys/abbey-style-ale-brewers; we were there for the sours.
