Lying 70 mi/113 km west of Stuttgart, Baden-Baden is the Black Forest's (and perhaps Germany's) most famous and luxurious spa. It has many fine old-world-style hotels where visitors sometimes spend weeks relaxing and "taking the cure" in the town's famed mineral waters. (Some of the thermal springs have water temperatures of up to 154 F/68 C.)
For a more family-oriented atmosphere, try the large and lively Caracalla Spa (bathing suits worn except in the sauna area). For something more upscale...