Strasbourg, France

Yes it can be done. Lovely town.

Yes, I visited Strasbourg as a daytrip from Colmar and found that adequate. There is a direct TGV train from Munich to Strasbourg at 6:27 Mon-Fri (check schedules for your actual date), and you can buy that ticket with an advance discount through the DB site. The next several morning departures require 2 - 3 connections. How to Look Up Train Schedules Online gives you the DB train schedule link and tips for using it.

If Salzburg is not already in your plan, that would be a much closer, recommended daytrip from Munich.

Tiny historic center - you can easily visit the old town, the wonderful Cathedral and take the canal boat trip that takes you out to the EU headquarters as well as through old town -- in a day.

You could spend only one day there, but it is not enough, in my opinion. What I liked about making Strasbourg my base for that region is that it is indeed "where all waterways" and roads meet-- where everything meets, including rail. We did day trips to Obernai, Colmar, Winzenheim, etc. Strasbourg's waterways are compelling for someone who appreciates this kind of engineering and feature, the town is fascinating, as well as the walkways around such. The cathedral there towers out of nowhere, has an amazing clock outside with moving features. Obernai is utterly charming. "Le Petite Venise," as they call it, is charming and romantic. There, we came upon an exceptional farmer's market, and took a walk on pathways between vineyards above the town. The old city wall is built into the houses. Colmar was lively actually, with jazz outside, art inside, friendly talkative people. We even saw an example of an Art Nouveau building. The trip to Wizenheim was so we could take the trail above the vineyards and into the forest, where we climbed Hohlandsbourg Castle, which had views of wine towns in the valley. I cannot recommend this area highly enough! And this was all by train.

As Laura mentioned, I've also taken a day trip to Strasbourg from Colmar, and felt that was quite adequate. If you arrive early enough, there's more than enough time to see the magnificent Cathedral, take a boat trip on the canals (which provides another view of the alternate E.U. Parliament), take the tacky tourist train for a brief tour around town and have a nice relaxing lunch.