I remember as a child using a sleeper ferry to get between Jersey and the British mainland. Politically Jersey is ours (it's not technically part of the UK but it's a crown dependency), but geographically it's basically in France. Seems like these days there is no overnight option but the long slow ferry from up the coast does take like half a day to get there and you can book a room so you can get some sleep.

I've taken the ferry from southern Italy(bari) to Croatia and back multiple times. It's a great way to travel. There's a camaraderie on a boat you just don't get on an airplane. Also helps that I can bring my car with me!

The Baltic has a bunch of overnight ferry routes too. Most of them are not very luxurious, but its a nice way to get both travel and sleep done in one go.