First off let me just say that the last cruise we were on was Royal Caribbean Oasis of the Sea and I had a two story suite that was fantastic. That being said my wife told me she had a great time but the ship was too much of a circus (climbing walls, mini golf, basketball, etc) not to mention the 8000 people on the ship. I was pointed in the direction of the Windstar Windsurf. We took two boys ages 17 and 22. The cruise was over Christmas so we figured (and were right) that there would be other young adults on the ship. It was fantastic, from the room (not as big as the last ship) to the food to the crew. I don't have enough time to leave a complete review but we couldn't have been happier. The kids even had a great time. We went swimming and playing in the ocean off the back of the ship every day, something we haven't ever had the opportunity to do before. This was like being on our own private yacht. It's a small ship with only 300 passengers and if you wanted to find a corner and read or listen to music it wasn't a issue.
You are going to want to take advantage of the 24 hour open bridge policy, it's fascinating. Everything we did was special and we would do it again and will.