MyriamC, Thank you. This is very helpful.

I'm trying to get it out of my head to spend at least one night in a smaller place such as Ghent but I don't want 3 different places out of 4 nights. So I have to work on myself a bit.

Thanks again.

We have 3 full days plus from about noon on our arrival day.
Right now we're looking at day trips to Ghent (been to Bruges twice over the years) and Antwerp.

My wife is a retired librarian so we try to visit libraries wherever we travel (obviously the beautiful and older ones). The Leuvin University library look very nice and might be of interest.
Leuvin is about 25 minutes from Centraal and might be worth going for a few hours. Any comments on Leuvin?

Antwerp is also a day trip possibility but I want to make sure it's a nice walk around (or bike around) destination and not solely a museum or a modern city.
Comments on Antwerp?

Thanks.

Arrival day for orientation walk in Brussels, than 1 day each for Ghent, Antwerpen and Brussels.

It's not a library, but as fellow bookworms you'll enjoy visiting the World Heritage listed Plantin-Moretus Museum in Antwerpen.

https://en.wikipedia.org/wiki/Plantin-Moretus_Museum

Antwerpen is a nice walk around, although it's quite large, even only the old town. It's not solely a museum or a modern city, it's a perfect mix of both.

Leuven is a pretty small city and the Uni library is a gem. https://www.visitleuven.be/en/university-library-tower
In Antwerp: the Hendrik Conscience Heritage Library, and especially the Nottebohm Room (you'll have to check opening hours). https://www.consciencebibliotheek.be/en
This library is next to the baroque Carolus Borromeus Church, that is also worth a visit.

Antwerp is very cosmopolitan. It has a fair amount of museums and churches and great architecture all around. It's not a very big city, everything is walking distance from Central Station. Central Station itself is worth a look. It's also called the Railway Cathedral and is among the most beautiful train stations in the world.
And I didn't even mention the excellent restaurants ...

We didn't find Antwerpen all that large, at least the areas we most wanted to get around to, including the old city. We found it very friendly, with a nice riverfront, great restaurants and cafés, a fascinating exhibit at the town hall about Spanish incursions, nice shops, and (important to us at the time, as we were suffering from too much European food) a lively Chinatown. The train station is magnificent.

We enjoyed exploring Leuven. Notes from my TR. If you time your trip better than we did you can probably include a tour of the Town Hall:
On the way to the Grote Markt/cathedral, we stopped to see the university library. The original library and much of the city was heavily damaged in both World Wars and much of the library collection was lost. The library building was built in a a neo-Flemish-Renaissance style in the 1920's with donations from U.S. universities and colleges. It was rebuilt after World War II.

At the Grote Markt we stopped to admire the exterior of the beautiful and elaborate Town Hall. Unfortunately we were a little too late for the guided tour of this building. We did tour St. Peter's cathedral with an audio guide. The building is an impressive 15th century Brabant Gothic style structure, but the cathedral is best known for its art collection. The chancel and ambulatory are a museum and visitors can view a collection of sculptures, paintings and metalwork. The two paintings by the Flemish Primitive artist, Dirk Bouts, the Last Supper (1464-1468) and the Martyrdom of St Erasmus (1465) are the highlights. My husband enjoys misericords and there were some good ones on the cathedral choir stalls. (Misericords are the small wooden shelves on the underside of a folding seat in a church and often have carvings of animals or monsters and grotesques.)

By the time we had emerged from the cathedral, the rain had stopped and it was time for lunch. We ate at a restaurant/small brewery called Domus in the center of town. We tried their Nostra Domus and Con Domus beers and had venison and something-else-I-can't-remember. There seemed to be quite a few students eating pasta and pizza dishes.

After lunch we walked around the Oude Markt, the university area and the large and well-preserved Groot Beginjhof. We didn't make it to the museum or the Klein Beginjhof or to some of the city's historic churches and could easily have spent more time in Leuven, but we had to get back to Brussels for an early pre-theater dinner.

We also enjoyed our day in Antwerp and found it quite walkable from the train station.

I think the Hendrik Conscienceplein is the most beautiful square in Antwerp. And as Myriam said, if you love books you cannot miss the Plantin Moretus museum.
You say end April/May; I believe I already mentioned that May 1st is a holiday. Museums will be open, but shops are mostly closed in Antwerp on that day.

BDKR, MyriamC, StCirq, Vttraveler,

This info is invaluable.
This tells me exactly what I want to know
Thank you. Thank you!!!!

The arrival day we have from about noon on and the last day we have all day until about 9PM. We'll be in Brussels and try to fit in half a day for Leuven. I think the library is open on a Saturday. Hope so. Otherwise it will have to be the day we arrive.
On the two middle days we'll go to Ghent and Antwerp.

Thank you one more time.