You can take similar excursions to other lakes in the alps, like Schliersee or Chiemsee.
== Moving on ... ==
As the next destination for your trip I recommend Budapest. There is a daily night train that runs from Munich to Budapest, which leaves late in Munich, and drops you off early next day in Budapest. There have been some changes lately in the train system, and to book tickets, you'll have to do this via the Hungarian train company. Unfortunately, their web site is only partially in English (they're better in German), so I recommend someone helping you with those tickets as soon as you know the exact date. At the latest, I recommend asking the Hotel staff in Munich to help you out as soon as you check into your hotel there. The train name is "EN 463 Kalman Imre", get yourself a cabin for two adults, and you also may qualify for a senior discount. The train leaves daily at 11:35PM in Munich, and arrives at 9:24AM in Budapest-Keleti.