If you're there beginning of Sept. up untill the 10th plan an afternoon/evening at the local fun fair "Herbstplärrer" in the city, which is the largest fair in Swabia.

I think a day or two for the city which is one of the oldest in Germany and has a lot to show out of the 1500 and 1600 times around the 30 years war and the times of the Fugger should be included.

If you're into lovely scenery plan for a day trip south by train or car. I would advise either castle Hohenschwangau and Füssen town or a trip via lovely Walchensee to Garmisch Partenkirchen. A bit farther (so probably rather a 2 day trip) would be the inclusion of the austrian Lech valley and Oberstdorf/Immenstadt for the Hohenschwangau tour or Innsbruck and Achensee or the Stubai Valley in Austria on the Walchensee trip.