Day trip from Brussels to Luxembourg or Cologne doable?

Cologne is well connected by rail, and very much walkable from the HBF. But 2 hours each way is too much for me. I say 3 days for all of Cologne; It's a lot more than just the Dom. Roman museum may be closed. Advance-purchase rail ticket might be much cheaper.

https://community.ricksteves.com/travel-forum/germany/can-i-see-enough-of-cologne

I personally would not daytrip to Luxembourg City.

Instead, visit Leuven, Bruges, Gent, Antwerp, Lier, Turnhout. ... . Antwerp has almost as much to see as Cologne.

EDIT: Also Aachen, closer than Cologne. I should note that there was some sort of problem with my return train to Brussels from Cologne, and we had to get off and wait for a replacement train, same track, in an empty, border-like station.

Cologne yes, the trains are fast, but Brussels - Luxembourg is a milk run and trains stop too frequently. The journey is around 3 hours, which for me makes it unviable as a day trip. Personally I like Luxembourg, but maybe make it a stop on the way to Trier, which IMO is also worth seeing.

Cologne is a good trip. There are a lot of good museums: the chocolate museum, two perfume museums, a museum of ancient Roman artifacts, art museums (e.g. Wallraf-Richartz), the Cathedral, a fabulous shopping precinct, and the brew pubs, serving the local Kölsch beer. The nightlife, if you are into that (I am not, I travel with kids) is also good. If you had more time you could pair it with Aachen, which is even closer to Brussels than Cologne. In Aachen you have the cathedral where all the German kings from Charlemagne onwards were crowned. A nice city.

Good luck, whichever way you go, I don't think you could go wrong.

Lavandula

The advantage of Cologne is (a) good fast direct train and (b) the Dom is right there as you leave the train station. Even if that’s all you do in Cologne, I think that’s worth a day trip.

A lot of the centre of Cologne is rebuilt in a historical style but I do recommend the Roman museum, which is nearby. Also I seem to remember some underground Roman remains but I may be getting confused with another city entirely.

We booked a Luxembourg day tour from Brussels this past December with Buendia Tours. It was a good day, but a long day. It also stopped in the city of Dinant. I would recommend if you want to visit Luxembourg you book a day trip. Just google Brussels to Luxembourg day trip, there are many options.

I would agree with others that Cologne would be pretty easy to visit by train.

In the end I would say that Luxembourg is kind of out of the way, It will almost always require a special trip. Cologne would be easier to work in to another trip due to the fact that is it is well connected, and that area of Germany (Cologne, Bonn, Aachen, Rhine river valley) can take many days.

Either way have a great trip you cannot go wrong with either city.