Munich for 1 day

Might depend a little on where you are staying, but I would say,

be in Marienplatz at 11 AM to see the performance by the Glockenspiel on the Neues Rathaus. Then see Marienkirche, the church near Marienplatz, with the green cupolas. Have lunch at the Hofbräuhaus. Tour the area. Go up to the top of the church tower at Heilige Peter. Go down to Isartor, one of the old gates of the city. Go the other direction, the pedestrian shopping street from Marienplatz to Karlstor. Near downtown is the Deutsches Museum, their version of the Smithsonian Museum in Washington.

Rick has a well-organized Munich city walk in his Germany guidebook. Most of the must-see highlights in the city center are covered. The map and audio tour are downloadable from this website here. Click the "Listen" tab.

You've had some good advice already. There are many more museums, depending on whether you're interested in art, cars, airplanes, etc., that people would see with a longer stay.