We once got each other off under a blanket in a room full of other people (who didn't know we were doing it). That's probably the strangest/boldest place. We also had some fun in a dressing room once, but that was pretty brief.
On the rooftop of an office building overlooking downtown of a major city at night. I took a blanket and spread it out, then had a picnic basket full of goodies and some wine. The sex was fabulous in those conditions. Due to having keys to everything in the building, I also took a date into an elevator one night and shut it off. Sex right there on the elevator floor for a long time that left great memories ever time I rode it in the future.