Leuven

The month of the year and the day of the week are always important in evaluating tourism decisions. Most formal attractions, like museums and historic buildings, close by 5PM all over the world. You don't indicate if your interests are primarily nightlife (or Belgian beer ... ) or something specific, like a Beguinage or historic exteriors of buildings.

I mentioned Beguinage because Leuven happens to have a very nice one, most of which is just as attractive after hours as during the day - the buildings are occupied so you can't enter them anyway. Leuven is a big university town, which means lower prices and lively street traffic. Brussels is a bit emptier after business hours, but it's a far bigger city, with lots more to do - as long as you have money for the Opera House or whatever.

Leuven had a big fire some 100 years ago, so it's not quite as historic, but the famous University Library is such an important building that it has been rebuilt (from war damage) TWICE. There's a superb smaller art museum, a lovely city hall, and a nice cathedral. It's a very attractive smaller city. If you don't care about the art museums in Brussels, I'd spend the evening in Leuven during school season. One hotel is a renovated monastery cloister, but there are hotels of all kinds.

Great information, Tim! I didn't even know what a Beguinage was. Thank you for enlightening me.
It sounds like we'll be better off staying in Leuven. I'd like to soak up the atmosphere of a college town during the evening. We can see the major sites in Brussels the next morning before our festival.

If you're going to be using a lot of public transportation (don't know what time the Festival performances are over, and if it's still running ... ) you might want to research which of the two public transit companies serve the three cities you're going to be in. (Most of Leuven is entirely walkable, but the Beguinage is a long hike without the bus. I have walked back to the train station from the Beguinage, but it's really far.)

I couldn't understand the advice I've read online, including here. But it appeared to me that Leuven (and, who knows, the festival town) MIGHT be part of the Brussels transit system, while all the other cities in the area can use the same card of the wider-ranging transit company. I just bring it up so you don't have to buy more transit cards than you need to. Some years ago, the busses I took were more expensive in Leuven than they were in Antwerp.

If university is in session, Leuven will be far more lively after dark, particularly the student cafes on the Oude Markt. If forget the name, but there's a pedestrian-only street near the Oude Markt that has several excellent restaurants...even by Belgium's already very high food standards.

I just wanted to follow up from my original post. Leuven was a total gem! It was so charming and beautiful. The evening atmosphere was very lively. I'm so glad we stayed two nights there.