a few years ago I took my wife for a weekend in GErmany - munich, berlin, dusseldorf. we spent the first day in munich, then the night train to berlin, then a night train to dusseldorf.

I would recomend the night train - I take it all the time for work, but for somebody who hasn't done it it can be a real adventure. it gets you right down town in berlin, early in the morning, and this way you don't waste any siteseeing time. the rooms are pretty comfortable, the big problem is that not all the trains have showers, so you either have to not care about showing in the morning or check into your hotel early (or use the nr. clean showers in the berlin train station, which isn't a bad option).