If you're coming next weekend, you can catch the tail end of the Oktoberfest. Otherwise, I recommend the usual touristy stuff in and around Munich:
- Neuschwanstein Castle
- The Pinakotheken (three well-known art museums - one each for old, new & modern art)
- The Deutsches Museum (the world's largest science & technology museum and my single favorite museum in all of Germany)
- Nymphenburg Palace
- The Residenz
- Take a stroll in the English Garden (one of the largest parks in the world)
- BMW World if you're into cars
- Guided tours of the city center
- Have a Ma (a liter of beer) at some of the many beer gardens (not in the city center, though, those are tourist traps - I recommend Hirschgarten or Hofbrukeller at Wiener Platz)