Hamburg is less than a 2h train ride away and boasts a good selection of museums and activities. Germany's best city hotel also here:

https://www.fairmont.com/vier-jahres...SAAEgLH2_D_BwE

And German billionaire Klaus Michael Kuehne's prestige project as well:

https://www.thefontenay.com/en/?gcli...xoCtisQAvD_BwE