Hmm, I like the idea of stealth sex. My bf and I have snuck off to make out at parties, but not actually had sex at any parties. That would be fun. We do enjoy finding random places to fuck in the car, though.
Also, a big fantasy these days would be to have a woman kissing me and allowing me to fondle her while my man is fucking me or going down on me... something like that. Hell I could even imagine a second woman maybe rimming me while he's inside, if that's possible... mmm I like that.