Sounds like you're going to have a great trip. Here's a couple of my favorites from a trip some buddies and I took touring the Belgium countryside and visiting breweries.
Cantillon - If you were going in the fall, they have an "open brew day" where they let the general public come in and watch them do a brewday, from the time they start the mash until they pump all that beery goodness up to the copper lined attic that serves as an open-air cooling vessel.
De Dolle Brouwers - Awesome people. Small family operation, like many breweries in Belgium, the hours are limited (at least they were where I was there). I quick look at the website indicates they're only open on the weekend. Hands down my favorite stop of the trip. The owner/brewer started pulling out bottles from his private stash that he wasn't exporting at the time for us to try. His wife also makes pate and serves it with a homemade mustard - very fresh, VERY spicy. Like I said, really awesome people.
Dupont - Ok, not the best atmosphere... not a small, family-run feel to the operation. But, we got there early one morning - like 8am. Lucky stars, language barrier, crew that didn't want to bother - whatever it was, they just let us roam around the brewery without a tour or an escort while they worked. That was certainly different. (Especially after spending a week doing enough brewery tours that you could give a tour in your sleep.)
Rodenbach was also interesting - although we went on a Monday and the town it is in was like something out of an Alfred Hitchcock novel. We couldn't find a living soul in town - it was eerie. Apparently Monday's are like that, with many business owners taking the day off after having their stores / pubs open over the weekend. I was there shortly after they were purchased by Palm, and most of the people we talked to were very concerned about if they would still have a job.