1. Castle 'Burg Eltz' along the Mosel river, can be reached by train connecting in Koblenz.

2. Luxembourg, if for no other reason than to say you've been there.

3. A drive along the Rhein or Mosel river at a leisurely pace, enjoying the small wine towns is an idea.

4. There is an airplane museum in Sinsheim, which has I believe both a Concorde and a Tupolev Tu-144 on display.

5. You could rent a fast car and test drive it on Nürburgring, but be careful.

All of these are close to where you are going to be.