The museum you can arrive up at any time, but for the tour of the factory itself you need to book inadvance.

You can hire most cars from the range:

http://www.bmw-welt.com/en/bmw_on_demand/

But if you want to get the M4 or M6 you should book in advance too. Hire it for 3 hours and you get to take the car for the day. If you get the notion drive to Stuttgart and visit the Porsche museum, you may have to overnight for that to make it doable though and get the enjoyment out of the ///M car.

A humbling thing to do is go to Dachau Concentration camp, I think everyone should see this place to understand the atrocities that went on. Short drive or train or bus from Munich:

http://www.dachautour.com/

Go to the HofBrauhaus too.

http://www.hofbraeuhaus.de/