Exhibitionism. In particular when a lone woman decides to get completely head-to-toe naked in a public place, indoors or outdoors. And by completely naked, I mean completely naked. Barefoot, no jewelry or accessories, nothing at all. Naked as the day she was born. She doesn't get caught and there's no sex involved (except the eventual sex with herself), but her fantasizing about possibly getting caught and the resulting ENF/humiliation only ends up getting her more and more aroused. Unfortunately such stories are hard to find.
I like stories about young lovers discovering sex - but too often they're about kids under 17 and we can't do that here. Another type I really enjoy is about older people reuniting after many many years, and perhaps after a few marriages.
I like stories about casual sex, steamy sex, "for the heck of it" sex, sex "just because there isn't anything else to do", etc.