Need day trip from Colmar...quick

Baden-Baden, Basel, Freiburg im Breisgau

Strasbourg is gorgeous but still in the Alsace.

You don't say your method of transport.

Car: Freiburg im Breisgau. Use a "park-and-ride". This is complicated by public transport from Colmar, bus + train.

Train: Basel. Head for the Münster (cathedral). Ask someone how (tram No. 2 from the station), then wander around the old town.
I would say go to the tourist office in the station, but they started building work there last week, and I am not sure where it is now.
Warning: it is hot here at the moment, and likely to stay so (~30°C)

We are flying out of Basel, Friday morning. Mode of transportation is bus/ train. We have just gotten our fill of pretty towns that don't have that much to do... and Nothing at night (last week we were in London with plays and late museums).

Tempted to take the train back to Paris. :-/
Thanks for the suggestions so far.

There's plenty to see in Basel. Kunstmuseum is a stand out among many Basel museums. No theater though unless you speak German.

Are you interested in cars, trains, or both? If so go to Mulhouse, which has the French national museums of railways and cars (separate), both of which are easily reached by tram from the station and big enough to make seeing both in one day a struggle.

Freiburg is not far away but probably hard to get to quickly by pubic transport. Strasbourg will be quick to get to, Baden-Baden maybe two hours maximum.
I visited the two museums in Mulhouse but found Mulhouse itself to be terribly drab.

I was in the area in May. I felt I'd seen what I needed to see in Alsace and had just come from Burgundy so I decided to wander for a couple of days in Germany. I ended up choosing Freiburg and going to the thermal baths, then spent another day in Heidelburg. If you want some nightlife. I think you at least want Strasbourg or Heidelburg. Baden Baden might have some nightlife but I didn't visit there.

See if there are any events going on in the area. There were a bunch of renaissance festivals in Burgundy when I was there but I seemed to miss them by a day.

If you haven't been to Burgundy, Dijon might be a good choice too.

Nothing in the area will have the nightlife of London.

Visited Freiburg today and had a grand time. Looking fwd to Basel tmw. Thanks for all suggestions!