Bad Krozingen is a small spa town in far southwest Germany, very close to Colmar and the border with France. You can also hop on a train from Freiburg central station, which will bring you there in about 10-15 minutes. I live in Germany and spend about half my time in the area of Freiburg im Breisgau, so this was a day trip for me.
The highlight of the town is the thermal baths, named Vita Classica. They use water from a local hot spring, basically the same idea as the much more famous baths in the town of Baden Baden. I've been to a number of thermal baths in Germany (though not Baden Baden!) and by comparison to most places, the complex is h u g e. Multiple indoor/outdoor pools, multiple indoor pools at different temperatures, an outdoor hot tub, and also a sauna complex (which I didn't try.) Some have areas for bubbles or circular pools with a current in them, some have spouts of water which pour into the pool, the usual thing. I didn't even have a chance to see all the different pools because after trying 3 or 4, I ran out of time and had to go. The really cool part is the carbon dioxide level of the water-- this spring has the highest dissolved CO2 content of any spring in Europe, and the result is that you'll have tiny carbon dioxide bubbles forming on your skin if you stay in the water for any length of time. With the colorful pool lighting, it looks absolutely trippy. Awesome. The buildings and decor feel a little haphazard and in places a little dated, though, so don't come expecting architectural marvels.
Admission to the Therme costs something like 25 euros a person per day-- cheaper than the baths at Baden Baden. You can leave and re-enter all day as you please. Unlike most German baths, the showers are in individual stalls with doors that lock, and I have yet to see people wandering around naked, so if you're put off by locker room nudity, you would likely be comfortable here. Signs are in German and (to a lesser extent) French, and I'm not sure how much you'd be able to get around in English. It certainly doesn't seem to be a major tourist destination for Brits and Americans.
After our visit to the baths, my travel partner and I wandered around the town. There's a small city center/commercial district around an old church and its associated graveyard, with far more restaurants (and international restaurants) than one would really expect for such a small town. We ate at a Thai restaurant called Nam Khan Sanee, which was delicious, absolutely the equal of any Thai restaurant I've been to in the US. Its prices were extremely reasonable for the quality; you could expect to pay 5 to 10 euros more for something similar in Freiburg and still have it not be as good.
All in all, a really pleasant way to spend a day. I think the baths would be worth visiting for the bubble experience even if you were visiting from far away, and especially if you were staying in Freiburg and wanted a German Therme experience and didn't want to make the trip all the way to Baden Baden. There's another Therme in Freiburg called Kedel Therme, which has nicer surroundings and architecture but is probably about half the size. The town itself was pleasant but probably not worth the trip on its own-- better to go to Waldkirch or Emmendingen if you want a pleasant small-town experience. But the food scene seemed to be top quality.