In case you don't have these links:
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-besides-bruges-and-ghent
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-768c7c85-75b9-4374-ad13-ae36967bc521
I don't know about post-Covid, but both Belgium and the Netherlands used to have an Open Monuments Day weekend in September, with access to places not usually open to the public. The last time I did it the paperback guide was only in Dutch, and only printed the month before.
Netherlands notes:
The very important KMSKA art museum in Antwerp reopens after 5+ years of renovation, I think in September, 2022. If you really like art museums you might have the opportunity to add Lille, France. Ostend may be a bit of a beerhall in the summer but is unlike the non-coastal cities in the same area. I went there to follow the trail of painter James Ensor. Artist Rik Wouters is associated with Mechelen. Of course the excellent open-air museum in Bokrijk is better in good weather. After a certain number of visits you have to decide whether you need to see another small medieval town square and cathedral. I mean like Tongeren or Hasselt, say.
Edit: Having said that about town squares, I should say a word about "How many Beguinages do you need to see?" Certainly Bruges' is the most famous, and quite big, but it's not my favorite. For one, it is too gentrified, by buildings redeveloped into gracious modern apartments-despite UNSECO WHS status. I rather like Leuven's (city bus ride from the real town center) and Turnhout's the best, because they have a real feel of an enclosed quarter, and are much less crowded than Bruges. Turnhout's has a museum, not open every day. Leuven looks reminiscent of Bruges', and has a nice church still in daily use, with a striking completely abstract modern Stations of the Cross. Turnhout has a senior center where you can get coffee and cake, outside the locals' meal hours.
I wrote that Mechelen's is a ghost of a Beguinage, and that's why you need to see a few well-preserved ones to appreciate the winding (and very quiet) but modernized street in Mechelen. You have to watch for remaining stone ornaments and Saints along the walls, for example.
Somehow, I have missed the Antwerp Beguinage, so I can't report on it.
I should also mention the Antwerp red-light district, Schippersstraat (That would be the name, wouldn't it!) near the MAS museum. I don't like the voyeuristic, sin-tourism aspect of red-light districts, but it was interesting to me how sanitized and silent the same street is all day. The Netherlands-ish street urinals simply don't exist until the opening hour, and the show-windows are invisible. Note: This Schippersstraat report is 6 or 8 years old.
Not saying it suits your needs, but there used to be an hourly bus from BRU to the side of the main train station in Antwerp. One stop, at a ring-road hotel.
Netherlands notes:
https://community.ricksteves.com/travel-forum/netherlands/amsterdam-to-brussels-to-brugge-back-to-amsterdam-by-train
https://community.ricksteves.com/travel-forum/netherlands/day-trips-from-delft