The Walldorf bathing lake is a real alternative to the Langener Waldsee. Smaller, not so crowded, somehow more relaxed (I used to have the whole lake to myself after it was still raining in the morning). On the southern shore at the entrance there is a large sandy beach with a snack bar, sanitary facilities, beach volleyball and a barbecue area. There are many trees to the east (hammock...), steps and paths lead down to many small bathing bays. FKK is popular in the north. And in the middle of the lake there is a small bathing island.
What can be annoying is the proximity to the freeway - you hear it all the time.
Bathing season is around the beginning of May - September, then the entrance fee is €2.50/€1.50 and there is a boat lifeguard. Outside the season, the lake is freely accessible at your own risk.
EDIT: In 2019, for reasons of liability law, it was reclassified from a bathing lake to a bathing area. Since then there has usually been no lifeguard supervision, no bathing island either, free entry except for parking. Somehow different now. 🤔