OP, if you forego a tropical beach resort, I have two more suggestions (besides Gotland) for exotic, charming, low-key, slightly off-the-beaten-path destinations:
Luang Prabang, Laos
Ljubljana, Slovenia
Of course, there's always Paris. Beautiful, romantic, and, yes, relaxing because it takes so little effort to be entertained there. Just sit in a cafe or walk down any street and there will be something interesting to see.