If you rent a car, there's an awful lot that's an easy day trip: Salzburg, Garmisch, Innsbruck, Nuremberg, Stuttgart, Berchtesgaden are a few ideas off the top of my head. I second the notion of visiting Dachau; it's a profound experience. If you like cars, a visit to the BMW museum is also worthwhile.

As someone else noted, it would be easier to help if you'd give us some idea of what your interests are. Munich is one of my favorite places and there's a huge amount to do there and in the area.