The Gaswerk in Neustadt an der Dosse is a technical monument and a museum. The facility produced gas from coal from 1904 to 1980 when it became obsolete. The gas was used to heat the houses of the town as well as run hot water boilers, fridges and other appliances.
The rooms of the keep of the Starkenburg above Heppenheim have the best view in the youth hostel (the castle is used by the Deutsche Jugendherbergswerk). It’s not the original keep, though.