Thank you LiL Team Member Claudia Foertig for this lovely tip!
My personal top tip for a beautiful badi and mountain lake is Seelisberg close to Emmeten. Lake Seelisberg is between the mountains with turquoise water. There is a playground, BBQ place, Kiosk, Beach Volleyball and toilets. You can rent deck chairs and rowboats. There is also a Camping site which you can use.
Distance by car from Luzern approx. 30 minutes. More Information can be found the official Website.