Where can I park my car overnight at a lake and sleep in it?

Hi, I'm looking for a nice, preferably quiet, secluded spot where I can park my car overnight and sleep. Ideally, near a lake, about 250 km from Stuttgart (Lake Constance, Lake Ammersee, Lake Chiemsee, etc.). I'm thinking of being able to jump into the lake directly from my car or within a short walk. :))….