AN: I've seen a couple of stories where Harry either talks about or does go bungee jumping off of Gryffindor Tower, but no-one ever fleshes it out, so I'm going to take a stab at it.
Disclaimer: I will never own Harry Potter.
It all started with two words. Two words that will forever be associated with fear and horror in Gryffindor Tower. Two words that should have never passed the lips of the Boy-who-lived, but did.
"I'm bored," said Harry one day at the end of May. It was a weekend. Harry, for once, had all his homework done for his fourth year classes. Hermione, surprisingly, still had a few classes to work on, and Ron, not surprisingly, has yet to start.
"You want to play a game of chess?" asked Ron.
"Nah, I'm getting bored with chess, especially since you win all the time," said Harry.
"Well, why don't you read a book?" asked Hermione, looking up from her Arithmancy schoolwork.
"That would just put me to sleep faster. No, I need some excitement. Something non-Voldemort related that will get my blood flowing," said Harry. Suddenly, an excited spark appears in his eyes. Leaping up, he shouts, "I've got an idea! But to make it work, I need to talk with the twins." Harry heads over to the corner where the Weasley twins are sitting with Lee.
"You know, the only time I see that gleam is when Harry's on a broom," said Ron.
"I'm beginning to think Harry's an adrenalin junkie," said Hermione. "The more death defying the stunt, the more he'll want to do it. Though I wonder what he's talking about with the twins."
"I need you to help me build a platform that extends about twenty-five feet away from the top of the tower," said Harry. "It also need to be structurally sound."
"Why do you need such a platform?" asked Fred.
"Have you ever heard of bungee jumping?" asked Harry. The twins and Lee nodded slowly, then looked at Harry in shock. "You're not..." started Lee.
"I am," said Harry. "I'm also going to need a jump cord, the ankle straps, a helmet, and an inflatable safety cushion, or at least the magical equivalent."
"That shouldn't take too long, especially if we get the house elves to help," said George.
"With luck, we should have everything ready by one o'clock," said Fred.
"I'll keep Hermione distracted while they work," said Harry. "Last thing we need right now is her going off about spew." The others nodded.
Everything went perfectly. While Harry managed to get Hermione to go to the library for research, (how, he still didn't know), the house elves built the platform in record time. The whole thing was secure and re-enforced to make it very safe.
The bungee cord was measured and tested with a Harry-sized and weighted dummy to make sure that Harry himself wouldn't crash to the ground. Once the right length was found, the cord was secured to the platform.
Finally, the time had come. Harry started on his way to the roof.
"Harry, where are you going?" asked Hermione.
"Oh, just up to the roof," said Harry as he headed on his way.
"I wonder why he's going up to the roof?" Hermione asked Ron.
"I don't know," said Ron. "I heard some construction going on up there. Maybe he was building something."
A shadow then appeared across the window. At first they thought it was a cloud, but two things abused that notion. One, it was a cloudless day, and two, the shape was too straight and angular.
Hermione went over to the window and looked up. There on a platform extending twenty-five feet from the roof, was Harry Potter strapped to a bungee cord. Hermione stepped back in shock.
"Hermione? Hermione, are you alright?" asked Ron, somewhat worried.
Hermione shook herself out of her shock. "Come on, we've got to get the teachers. Harry's going to get himself killed."
"What?! What's he doing?" asked Ron, now panicked.
"Harry's bungee jumping!"
Ron paused, and, with a confused look, asked, "What's bungee jumping?"
Harry was all set to take his first jump when he saw the teachers, as well as most of the students, gather on the ground below.
"Harry Potter, you get down here right now!" screamed Professor McGonagall.
Harry just smiled. "Okay," he said, and jumped off the platform.
The crowd gasped. "That wasn't what I meant," whispered Professor McGonagall, her right hand clutching her chest.
"YAAAAAHHHHHHH!!" the crowd heard Harry yell as he fell to the ground. As he fell, the cord he was strapped to became taut, and then stretched. As his head came within a few inched from touching the airbag, the bungee cord snapped back, flinging Harry back up into the air.
"YAAAAAHHHHOOOOO!!" screamed Harry as he bounced up and down like a yo-yo. "That was fun! Haul me up so I can do it again!"
"NO!" bellowed McGonagall. "You will unhook yourself from this contraption and have a detention with me in my office!"
"Aww," groaned Harry, but before Harry could unhook himself, a very familiar voice shouted out above them.
"Yahoo!" shouted Professor Dumbledore as he leaped from the platform.
"Professor Dumbledore!" screamed Professor McGonagall, who started swearing loud enough to peel the bark off of the Whomping Willow.
Thanks to Professor Dumbledore, Harry didn't have detention. However, he was asked to dismantle the platform, which he did, but he kept it shrunk in his trunk. After all, he never knew when he would get bored again.