Done the construction site, too, but I'm an architect, so y'know... access.  Also top deck of a mall parking lot on Sunday afternoon, graveyard at night, stairwell of a swank condo building during a New Year's party...  Had a threesome once in a private sex club with some rando watching me and my GF with our friend, but is that naughty?  The place is FOR sex.

I've done the playground at night, but best is in the walk-in freezer at work while we were working.