Don't worry about the speedless autobahn part, if you drive around Germany you will be on many autobahns without speed limits. They are usually in the country sections, with speed limits around cities -Germany Autobahn Driving Get a car thats at least up a notch from the economy level so it has a decent engine. Get a diesel. But make sure you put diesel in it.
From Berlin to Frankfurt you could take the southern route. Go to Dresden and Nuremburg, follow the Castle Road , stop at Rothenberg ob der Tauber, over to Heidelberg, then up to the middle Rhine. Neuschwanstein may be a little south out of your way. But there are plenty of other castles to see. Some with hotels you can stay at.
Castle Hornberg on the Neckar
Castle Guttenberg
Burg Reichenstein Haunted Rhine Castle Hotel
Take a Rhine Cruise from Mainz. Go one way and take the train back to your car. End up at Frankfurt. Some other thoughts. Check out the spa at Weisbaden. Your boyfriend might like go karts or a Race Taxi ride at Hockenheim Race Track near Mannheim.