Is Benelux in December possible/pratical?

Winter in Europe is just "different" and colder. It also is dark for more hours each day. Unless it is raining buckets, you can always do some sightseeing. Just expect and plan more indoor activities- which means Museums, theatre, pubs, concerts etc. Also more to do in the larger Cities. Amsterdam and Brussels are large sophisticated cities. If you are doing day trips- just expect to get wet, bring a good umbrella and good watertight shoes

Despite the popular perception in warmer parts of the US, northern Europe doesn't "close down" in winter. But as the others have noted, you have to alter your plans and expectations. In NW Europe, I have rarely found winter temperatures particularly cold. The biggest difference, though, are the very short days, low illumination and frequent heavy morning fog. If you're not used to this, it can come as a mental shock and delay your jet lag recovery. The practical implications are that you can't see and do as much outside of the cities. But if your itinerary was city-heavy, it shouldn't change much except your packing list. Both Belgium and the Netherlands both have wonderful Christmas markets. And actually, the most elaborate Kerstmarkten I have ever seen are not in Germany, but in Hasselt and Leuven in Belgium.

We recently visited Amsterdam, Brussels and Brugge, and it was quite cold to us, as we live in a dry and temperate climate. Based on a comment from the desk person at our hotel in Amsterdam, it may have been a cold spell - she said it was colder than it is in the middle of the winter. Anyway, we chose those locations because there were so many indoor activities. The one with the most outdoor focus was Brugge, especially as the Groeninge Museum was closed when we were there. But even there, it rained on and off, so we just always kept our umbrellas with us. Many cafes still sit outside, depending on the temperature; some have heaters, which helps.

Kat It sounds like you have specific "destinations" in mind. Benelux is a bit vague. What did you have in mind?

It shouldn't be a problem at all! I visited Amsterdam in late Nov/early Dec, 2007. One of the most wonderful things I found out was that everyone is still as lively and outdoors as much as any other time of year! Really! I was pleasantly surprised. We're college students and walked around very late and there were still tons of people out and lots of street food to eat! Amsterdam is really a very lively place. I haven't visited anywhere else in Benelux though, but I imagine visiting cities shouldn't be a problem.
I'd certainly bring rain gear and rain boots. As long as you take a few precautions, you'll have a great time, the rain won't stop you, and you'll be exploring beautiful neighborhoods while unprepared travellers are huddled and shivering underneath one umbrella. ;)