You can go easily by S-Bahn and just leave the train in Starnberg. The station is located directly at the lake. If you go to the lake and turn left you will reach after a few hundred meters a place called UNDOSA. They made a place there like a beach and serve nice food and good drinks. Fantastic place.
Furthermore if you like to make a picnic you can walk further, leave Starnberg and will reach soon the largest baech area of Starnberger Lake --- called the paradise.
Just enjoy. It is really worth a daytrip.
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