It is amazing how many stories come from unpredictable play. It is so incredible playing with players who have less preconceived notions about what should happen (i.e. the stories about the kids, etc.).
I have a few good ones that happened over the past year.
In one game, I ran a modified portion of a game for my 9th grade classes. The students had to escape a prison they had magically woken up in. Well, the PCs made it out of containment fields in one room, down a well after getting smashed into by moving statues that they escaped from, down into a cavern room, where one of the PCs found a small nook that had a box in it. The students were playing each pc as a group so there were 6 pcs and each was controlled by a group of 4 or 5 students (depending on the class and the number of students in the class). In one particular class, when the rogue PC opened the box, she found a potion. The group controlling her really wanted to drink it, and one girl in particular seemed very eager. When they told me that the PC drank the potion, I looked right at the eager girl and asked her to tell me the first animal to pop into her mind. With a giddy giggle and a smile she said, "a bunny." That's when the PC turned into a bunny. The group loved it, especially that one girl. They had the PC hop ahead of the rest of the group into the cavern toward the other end...heading toward three doors. One would lead to freedom, but the other two into more mayhem. (There was more, but the bunny part was the most adorable).
In a more serious campaign of Princes of the Apocalypse, my regular group was only 3rd level, but they had gotten into and down to see Queen Aerisi in the Temple of the Howling Hatred. They were damaged a little, and didn't have too many resources left, but they got Windharrow to take them to Queen Aerisi in her throne room. One of the PCs, an Air Genasi sorcerer, had a background I made up. He had to investigate the claims that Windharrow was some how responsible for the theft of a horn that was a sacred relic of their tribe. Well, he saw the horn in the Queen's throne room attached to the wall behind her throne. While the group was interacting with the Queen, and the bard was flirting with her because he seemed enraptured by her beauty, the Air Genasi slowly moved his way closer to the horn. All this time, the Queen was accepting the bard's compliments and slowly beginning to trust the group. The Air Genasi sorcerer could not let the horn just sit there. As soon as he was close enough, he took it off the wall. As soon as he did, the Queen called to her minions and the fight was on. The group killed some of the easier targets, but the Queen and Windharrow escaped using a dimension door. Then when the group got to the downstairs room, they were ambushed by a ton of cultists, Windharrow and the Queen. They had no prayer. That's when the Air Genasi decided to blow the horn, not even knowing what would happen. It summoned a Djinni (as per the module) and it allowed the Air Genasi to command it to do one service. The Air Genasi asked it to escort them back to Red Larch and defend them so that they could make it back safely. This turned the battle, allowing the group to take Queen Aerisi as a prisoner as Windharrow made his escape. It was a great session, very exciting, with great roleplaying, combat, near defeat, a sudden boon and a happy ending.