Once I peed in a cup while riding with friends cause we couldn't stop, then I tossed the cup's content out my window to just see it come back the rear window and splash the guys in the backseat. They were too drunk to realize what it was but I slid down my seat just in case they decided to pummel me senseless.
When I was in high school and just out of it, several friends of mine and I would take serious road trips. I remember we would get the old 36 ounce Coke bottles and a funnel. There was a rusted hole in one car's floor boards. When we had to get rid of the soft drinks we had rented, we'd put them into the bottle with a funnel. When full, we'd put the funnel into the hose and drain it out under the rear bumper.
I remember once when we had some guys following us, for whatever reason, and we couldn't shake them. I had to go but the bottle was full so I drained it. Clear night but the car following us had to use its wipers.