The Harwich/Hamburg one is brilliant - I've done this on about half a dozen occasions with my rugby team. Hamburg is excellent, it's got the Reeperbahn (not to be missed as a, ahem, tourist attraction - the red light district is better than Amsterdam), fantastic bars and clubs, culture, and if you go at the right time of year, it has the Dom, a big fun fair. Excellent. You actually stay on the boat, which is a 5 minute cab fair form the docks to the Reeperbahn, or a twenty minute walk. Be warned, depending on the time of year, it can be bitterly cold.