If you are near Hannover drop me a line here

(There is a small nice aviation Museum in Hannover-Laatzen).
Apart from that, depends what you are interested in. E.g. the Schiffahrtsmuseum Bremerhaven is a must-see if you are up there

There's a lot to see in every major city.
If you are into nature the 'Elbsandsteingebirge' in the east, or of course the Alps are nice. Lots of other regions though, the 'Harz' is also very nice, comes with a lot of mining museums and even a 'scotch' wiskey distillery. North sea, baltic sea, whatever ...
Have a nice stay,
thanks and greetings,
Catfish