Around Chiemsee, a lake in Bavaria. …..drove around it on Thursday. It’s bloody massive. Nice panorama of the alps. I’m assuming you are thinking on site. Lots of camping places around. Nice and flat to cycle, but with proper hills nearby. Day trip to either Salzburg or Munich, both around an hour drive.
Around Füssen, or rather near that massive Disney castle, there are lots of lakes with a few Campingplatz and again a mountainous backdrop. Kids can ride on the flat, you in the hills. Lots to do in the area…. The castles, cable cars, Munich is anb hour away.
Bodensee/Lake Contance is really f-in big (lived in Lindau for a year). Loads and loads of camping. Not much in the way of hills and no mountains on the German side (unless you are in Lindau). The Swiss side of the lake demands Swiss prices, but you have a nicer view from the German side.