Basically I think many museums in Munich are worth seeing but I like Residenzmuseum (the Antiquarium is definitely worth visiting).
At the Residenz, there are so many wonderful buildings such as Schatzkammer, the courtyards, theater and halls. Hofgarten is next door.
If you have one day to spare, you could happily go around all the areas within Residenz and pop into the National theatre (almost next door) in the evening to see an opera.
There are some boutiques and cafes nearby, so if you get bit peckish, you could always pop into a cafe for a cup of Milchkaffee. Or if you get bored, you could go to Maximilianstrasse to do some shopping.
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