I'm working on plans for my first visit to Belgium this summer. I am thrilled about the country-wide museum pass called (bilingually) MuseumPASSMusees. I don't recall seeing this pass mentioned on the forum. It's an annual pass costing only €65. It can be purchased at the participating museums, of which there are approximately 250! It's almost unbelievable; I think it's an even better deal than the Finnish and Estonian passes I used on earlier trips.
I haven't checked all the museums I'm interested in, but I've checked many, and the vast majority are covered by the pass. (I haven't dug into the policy about coverage of temporary exhibitions.) The major exception I've encountered is the Royal Museums of Fine Arts in Brussels, which aren't covered.
In addition to the financial advantages this pass offers since I'll be spending multiple weeks in Belgium, the process of reviewing the list of covered attractions has tipped me off to places I hadn't previously uncovered, like the tapestry museum in Tournai and the ceramic museum in La Louviere (outside Mons).
Note that this is primarily a museum pass. It does cover some historical sites, including Kazerne Dossin and Fort van Breedonk in the Antwerp area, but I don't think you'll find many castles, cathedrals, etc. on the list.