Bathroom? In the toilet if I want to be mess free, or in the shower, if I want spray cum all over my stomach, chest...and sometimes face.
Bedroom? Laying on the bed, stroking with my hand, the other hand teasing my nutsack, shooting on my stomach, and either wiping up with my boxers...or letting it stay there for a bit, the cleaning up in the bathroom. If I'm in bed, I'm using fantasies to get me off.
Computer Desk? Jerking with my left hand and lube, using the edging technique, cause I'm either jerking to porn, to and IM conversation, or on cam.
Outside? Have only done this a couple times, but have used spit as a lube, without get naked, and jerked through my unzipped fly.
In a car? Never tried this yet, though sex in a car is a different matter (though not entirely comfortable). I've been jerked in a car though...
In a public washrooom? Never did it at the gym yet, but have gone it will sitting on a public toilet with my dick between my legs. I rubbed the top of the shaft with my hand, while rubbing the head of my cock between my legs, and had a great orgasm this way. I've also jerked off standing over a public toilet while reading the "interesting and enlightening" reading material that others left on the walls of the stall.
In a hotel: Either in bed, so I can cum on the sheets and not have to clean up...or I jerk in front of the bathroom mirror and shoot on it while watching my face experience the orgasm