I can easily recommend Ypres. Public transportations are perfect for a day in Ypres. the Flanders Fields museum is situated on the market place of Ypres. I don't assume there's a train from Brussels to Ypres, but you can easily get there over Kortrijk. (Brussels-Kortrijk.. Kortrijk-Ypres)

Gent is also very nice, more city than Bruges, something between them. (don't have much experience on Gent, only been there few times, no personal bond) Antwerp is nice, lot of shopping streets, musea, old buildings (the train station, kathedral, vrijdagmarkt) You could visit the harbour of Antwerp on a boat. See the diamond quarter (lots of Israelic and Indian people)

Other sugestions?

A day at the seaside in Oostende, in Belgium called 'queen of the seaside city's' Nice to take a walk, see the James Ensor museum or go shopping...

Go to Doornik (Tournai) in the French speaking part. Really nice town, beautifull church, cave:'la merveilleuse', boat tours on the river Maas and of coursethe citadel (ruins of an ancient fortress) Totally different landscape than the Dutch speaking part of the country.

Ever heard of Bokrijk? It's an open air museum with old rebuild houses. Nice to wander around for a day. (don't remember exact location... I'll browse around for a minute, be right back...)

I am all for Ieper, you must go there!

check this out:

http://www1.limburg.be/bokrijk

This site can be translated in English, don't worry.

I would recomend to do cologne as a stop on the way to Frankfurt.

Hope to have been helpfull, nice greetings

Wendy

Made mistake yesterday, about the french-speaking part, I meant Dinant, not Doornik (Tournai) Oops, sorry...

1/ Yper is fantastic and can be done as a day trip. The museum is so moving, it is a must for everyone. Would be great if you could catch the last post at the Menin Gate. We were there last year, I had tears in my eyes.

2/ 2 Nice smaller towns close to Brussels are Mechelen and Leuven. both about 20 minutes by train. They are lovely towns with beautiful old buildings, many restaurants, nice squares and good shops

3/ I think Ghent is a bit similar to Bruges but Antwerp is something very different. It is vibrant and cosmopolitan.

4/ I would not do cologne as a day trip

for a short day trip with minimum travelling time i would pick leuven. bery easy to get to great bars restaurants and shops and very lemish and a big unversity town so it is pretty lively.

Thank you all for your very thoughtful answers.