One advantage of Mechelen and Leuven is that they are so easy to get to. They are both very attractive in their own ways, but not as rich a destination as Antwerp or Gent. The "ghost" of the Beguinage in Mechelen is kind of subtle, which is why I think it's good to visit an actual one before following the one in Mechelen. The interior of the Leuven city hall is only visited by TI tour, on Sundays, once a week.
Check museum open days for Mechelen. I've never found the Jewish Museum open. I'd like to see the mansion/art museum because artist Rik Wouters was from Mechelen. As a matter of sightseeing, you can take a bus to Mechelen from Antwerp and return by train, if you wish.
Returning to the Netherlands topic, we stayed in Arnhem, but had a car for daytrips. It's easy enough to visit the Palace in Apeldorn, but you need a little planning to get to De Hoge Veluwe park and the Kroller-Muller museum. Or small sights like Deventer - probably too hard to get to by public transportation, but a lovely historic center.
Mostly contemporary art, and you have to check the post-Pandemic schedule, but there might be a major art fair in Maastricht, called TFAF. I've never been. Because you mention April-May, have you been to Keukenhof?
