I organised a mate's stag do in Munich in April last year.
Get out to Kultfabrik, its an old factory in the east of Munich that has been converted into dozens of bars and clubs (there is even a strip club as it's a stag do!).
If you like football take a look at the fixtures and see if you can go when 1860 Munich are at home, no problem getting a ticket unlike Bayern. Watching a match is so much better than a stadium tour.
If you go late April/very early May Fruhlingsfest (Spring Beer Festival) will be on. This is like a smaller version of Oktoberfest, with beer tents and a fairground. We had a great time at last years.