Considering the Seehotel Maria Laach in Koblenz?
A VT member wrote the following comment about visiting Koblenz:
Ehrenbreitstein Fortress by RhineRoll
Visiting Ehrenbreitstein should be part of every visit to Koblenz. You can even stay within the fortress premises, because it contains a very nice (wheelchair-accessible) youth hostel. You can walk up the hill from the city, but there also is a large parking lot on the plateau-like hill outside of the fort. Mobility-impaired people will even get permission to drive into the fort and to the hostel's parking lot pictured here.
Visiting hours for the installations and museums roughly from 9 AM to 5 PM. Reasonable entrance fee. Decent wheelchair access, including loos, but some steep ramps within the premises.