Could you suggest cheap father-sons holiday ideas over the Sommerferien?

Hi all, I'm a broke half-time dad with two sons 8 and 10. I'll have the kids for 3 weeks over the Sommerferien. I would like to do something with them over the school holidays which is not sitting at home. Over the osterferien we ended up taking small bike trips, hiking or watching movies at home. I would like to do something more fun over the summer but have a limited budget. By starting to save now, I can optimistically come up with 500€ for 2 weeks and we'll spend 1 week sitting around at home.

Last summer we did a biking and camping trip which the kids enjoyed very much. The costs were kept in control also because we were invited to a wiesenfest ( camping party) with kids for 4 days. But that invitation won't happen this year.

I hope that by starting to plan earlier this year, I can manage to organise something that's value for money that the children will have fun with. I'm based in Baden-Württemberg and have a bahncard 50.

Could you please suggest some holiday ideas that are not too expensive? Or perhaps nice tours (biking /hiking/camping) we could try?

In return for your ideas you'll have our heartfelt thanks and a postcard from wherever we go.