I visited Belgium a few months ago and had a blast. I can recommend a few of my favorite experiences:
In Brussels, be sure to visit Cantillion (near the south train station). They make some of the best lambics in the world. You get a tour of the brewery and a free tasting at the end. For bars, you should go to Delirium. Their beer list looks like an encyclopedia. Another great spot was Bier Circus.
If you make the trip out to Westvleteren, I would recommend also visiting De Struise in nearby Oostvleteren. They were the only brewery I visited that has embraced US-style experimental microbrewing, and their beers are phenomenal. Another good one around there is De Dolles in Esen.
If you are in Antwerp, check out Kulminator. It was unfortunately closed when I was there, but I have it on good authority that it is the best bar in the world.
Have a great trip!