Another funny scenario is when you pack way too much food for a road trip and then things start to spill or get squashed. Like when you have a big bag of chips and suddenly the whole bag explodes all over the backseat. It's a mess but also really funny in hindsight.
There are times when you make a wrong turn and end up in a really strange place, like a street full of really odd - looking houses or a field with some very curious - looking animals. It's unexpected and hilarious. For example, my friends once ended up in a place where there were alpacas just roaming around freely and they were so surprised and amused.
One funny thing is when someone in the car tries to sing along to the radio really loudly and completely out of tune, making everyone else in the car burst into laughter.