Although it is nicer to sleep there, The Hague and Rotterdam (and Delft, tram ride from The Hague) are technically able to be daytrips by unreserved local train from sleeping in Amsterdam. I have been to Delft twice from Amsterdam, with different guests.
I don't consider Belgium to be a daytrip from the Netherlands, but there are many ambitious travelers on this board. Have you got or seen Rick's books about these places? They're very useful.
Some posters here, myself included, find Brussels a bit overrated, except for the three or four top art museums there. So I have always visited Brussels from Antwerp. Local unreserved (Belgian) trains make the entire area, from Mechelen to Bruges, easily accessible for daytrips. Many of these cities have business day trains five per hour. However, they are commuter cars, with high passenger loads, no real luggage racks for big bags, and sometimes lines for both ticket booths and ticket machines.
There is a lot of love for Ghent on this newsboard, but Antwerp is my favorite place to stay in that part of Belgium. Can you fly home from BRU? It would save retracing your steps.
I don't know what "reasonable" prices are. Antwerp has more lodging choices than Ghent, but some longer distances from the train station. We stay at the Hilton in Antwerp, but it's one of the higher priced options. Although both the Netherlands and Belgium have excellent city public transportation, it can be convenient to be able to walk to the train station for a daytrip.
The Search box top center will probably yield some more Antwerp suggestions.
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-768c7c85-75b9-4374-ad13-ae36967bc521
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-besides-bruges-and-ghent
Warning; There is some obsolete information in the next thread, but it may give you some ideas to research:
https://community.ricksteves.com/travel-forum/netherlands/amsterdam-to-brussels-to-brugge-back-to-amsterdam-by-train
Edit: I think this is an advanced topic, for those on a second visit to one of these countries, but I believe that Open Monuments Day 2024 will be Sept 14-16. But sometimes most of the publicity is in Dutch-only. This is like Open Day NY or Open Day Chicago, with normally private businesses or civic buildings allowing timed visitor tickets.