We enjoyed the Open-Air museum in Molfsee, right on your route, I believe. House, farms, a school house, church, from the area moved onto a park-like setting for viewing. On the weekends, there were demos, farm animals, and cultivation to see. Food on site. Big area, perfectly appropriate for kids to run and stretch their legs. Easy parking. Probably longer than a "pit stop", but an hour would be interesting even if you can't see everything. English language explanations of the families who lived in the homes and how the buildings operated.
https://freilichtmuseum-sh.de/en/startseite-neu
If you detour through Kiel, by the Labor Naval Memorial, there is a submarine that can be visited. The memorial is moving and views from the tower incredible.