Depends what you want.
If flat and easy, then Usedom in the North East was fun and novel.
It’s a traditional seaside resort area, so some nice towns, lots of sandy beaches, strandkorbs and interesting history.
Variety of accomodation from camping to hotels and everything in between.
Relatively cheap too.
Train accessible, and easy to take bikes on board, either to get you there or cover longer distances during the stay.
It’s old East Germany, and not a lot of the Queen’s is spoken by locals, so your daughter’s German (and yours) will likely improve a lot.
Peenemunde, home of the doodlebug V1s, is close by, and I had a great day out there. As did the kids – honestly!
Definitely a traditional (East) German holiday experience. Which may or may not be to your liking.
Garmisch / Munich it is not.
But I’d still recommended it!