The most relaxing vacation I have ever been on is a cruise. At least the days when we were not in port. When you are at sea and literally cannot go anywhere because you are on a boat there was no expectation or feeling of needing to be "doing something". You could just wander around, take a nap, stare out at the water, eat, read a book, play a game, etc. So incredibly relaxing. I'd love to go on a cruise that just went from point A to point B without going into port anywhere until it was time to exit the boat. That would be incredibly relaxing I imagine.