I was just going to suggest both Haithabu and the Sand Sculptures! The former is a pit stop (though Billund to Lübeck is an easy drive, so anything in between will also be easy as a side trip from either location). The latter is right outside of Lübeck.
Depending on your route you can also pass through Bad Segeberg and see the caves there. https://noctalis.de
Near Lübeck is the Dorfmuseum Ratekau. I haven't been, but it's one of those open-air, see what life was like back in the day places. Small, but free!
https://www.dorfmuseum-ratekau.de
Also an option as a stop is Arche Warder, a sort of museum / sanctuary / petting zoo for rare domesticated animals. You can feed rare goats, pigs, etc. like the Bulgarian Screw Horn Longhair Goat. We enjoyed it! www.arche-warder.de