I'm sure my answers are pretty tame compared to some here.
For pure crazy, I've have to say the old classic: changing room sex. Harder and harder to do these days, with all of the surveillance cameras everywhere - and I wouldn't necessarily recommend it, since it was kind of cramped and we definitely couldn't take our time.
The next most crazy place, was on a picnic table in a park, under a pavilion, during a rain storm. Now that experience was exceptional.
Same girl, both times. She had a bucket list of all the things she wanted to try before the cancer got the better of her, so we tried them all together.