I have two funny flying-but-got-to-go stories, one told to me, one involving me...first the told to me:
A flight instructor friend of mine was picking up a Cessna 152 that his flight school just bought, and flying it from Florida back to Michigan. On his first leg of the flight, he finds himself over the Everglades and needing to use the restroom. He realizes that he is nearly 30 minutes from being anywhere on the ground, and he doesn't think he can hold it.
In the plane, he has an empty Mountain Dew can (which is probably why he has to go so bad), and he chooses to "fill it up" while in flight. He then realizes he doesn't know what to do with a full Mountain Dew can, and decides to pitch it out the window, into the Everglades. When he tries to push the can out the window into the slip stream, his littering-karma was instant as the can didn't make it out the window, but instead falls back inside, spilling the contents all over the newly purchased plane. He said it was really hot that day in Florida, and the plane reeked of urine all the way back to Michigan.
My own personal story: In the early 90's I had just purchased a beautiful Rutan designed canard, and took it on a maiden voyage with my (now ex) wife to a fly-in about 200 miles away. It was a Sunday, and it was back in the days when buying fuel was always done in person, and required an attendant. Late on Sunday, on our way home and only 30 minutes from landing, my wife says that if we don't land immediately, she is going to pee in my new plane. I told her that we had enough fuel to get home, with reserve, but nothing more, and if we land, we would not be taking off until Monday because of the limits on buying fuel. She told me to press on, but go as fast as we can.
I found out just how fast that Rutan canard was, and I made it back in 22 minutes. Even then, the entire way was a two sentence conversation between her and I: Her, "I'm going to pee in your new plane!" Me, "Don't pee in my new plane!"
When we touched down I braked hard, shut down the engine as I braked, and opened the canopy as we came to a stop. She jumped out and pee'd right on the runway, even though there were two planes waiting to take off (which they thought was hilarious).