Your itinerary would be a little difficult as the Swiss Alps are very difficult to get through from Munich to Zurich to Lake Como. Zurich is out of the way--and it's not even in the mountains. And returning a German rental car in Italy will be very expensive.
We love to go out of Munich to the west and down to Western Tirol and the Innsbruck area by rental car. The Alps there are absolutely incredible It's a couple of hours back up to Munich where you could return the rental car.
Then take a 2 hr. train east to Salzburg for a couple of days. It's easy to take a train Salzburg to Vienna--another fabulous big European city. It's a place you could spend 4-5 days in easily. I know a great place to stay in the Vienna center city.
If you want to visit Italy, you could easily take a train down there from Munich to visit either Venice or Florence--or both. And then fly home from there. The trains are fast and frequent in Italy.