Bruges (Belgium) - Wonderful Venice-like, words fail...
Trier (Germany) - Very Roman-historical, Arena, Bathinghouse, Cathedral (organ)...excellent wine-bars...
Düsseldorf (Germany) - Stuffed with modern art musea and modern-art gallery's
Orange (France 'southern France') - Exceptionally charming and very musical...
Berlin (Germany) - Evening and nightlife...
If you permit me to give some advice, make a exceptionally good planning of your trip and try to stick to it (a little bit like 'fingering'), in order to avoid to be stuck in some station in no man's land, to avoid trains that stop every mile in... no man's land, to avoid to lose hours in trainstations waiting for the next train etcetera... The train is probably the finest way of travelling, but at the same time it can sometimes get deeply boring...
Kindly