I went on a College trip there, which also coincided with my 18th Birthday. I liked all the old style Bier Kellers, the one where Hitler tried the 'Munich Putcsh' was good - but expensive (the locals told us it was "die tourist trap").
I went to the BMW museum, which was OK, but nowhere as near as good as the Mercedes museum, which is 'up North' in Stuttgart. We went to Neuschwanstein Castle, which is an amazing sight to behold, but a couple of hours away by train.
Can't remember much else (I'll blame that on my 18th)... if you like um-pah bands, you'll love Munich.
I enjoy just wandering around the city, there's plenty to see.
Other than that, get yourself into any of the good Kellers (and not, upon almost pain of death, the Hofbräuhaus, unless you want the ultimate touristic experience).
Can I just say I'm very envious of any overseas trips.
I was due to be driving to Antwerp for a lovely Belgian beer-fuelled sight-seeing trip of the Low Countries, until we discovered this morning that the passport belonging to the half of our loving partnership that isn't me expired a year ago...