Do you have any suggestions for a solitary vacation?

I have a week of vacation available to take (the first in a long time) and I plan to travel somewhere by myself. I’m completely open to any type of location, climate, etc., although since I don’t have a passport, wish to make plans soon and have a short time frame to work with, I will have to limit myself to the U.S. and its territories

Have you ever taken a solitary vacation? Is there any particular place you recommend? Do you have any suggestions regarding solo vacationing (pitfalls, advantages, things you didn’t think of) other than the usual safety concerns? I am not interesting in meeting anyone, since I am in a relationship, so a “singles” vacation probably isn’t appropriate. I want to spend my time relaxing and enjoying myself. I am particularly fond of historic sites where I could immerse myself in the period, striking natural landmarks and natural beauty in general. I enjoy being around the water as well. I want to relax and not have to do any work, so camping or hiking (other than a day hike) would not be the direction I’d go. Any ideas?