Each year I go on a city trip with some friends, within Europe. We go around five days, and in recent years we've started visiting several cities in that time.
We have a new idea: to cut down on the time lost travelling in between cities, we could use night boats. They add something special and move us while we would otherwise be asleep in a hotel.
Example: in 2015, the current idea is to fly to Napoli, stay around there a bit, then take the night boat to Palermo and later fly back from there. The boat leaves around 8pm and takes 10 hours, so we have to get up early but otherwise it's a pretty OK night.
Between which other European cities could we do this? Both sides would need an airport in the region, the boat has to leave in the evening, take all night and then arrive in the other city.