I'm still thinking the story will take place towards the end of book 3, between 12 am and dawn just after Sirius and Buckbeak are saved, while Lupin is still a werewolf. It will draw on characters from the first 3 books. Here's how I see the ride:
We board thestral-drawn carriages (although the thestrals aren't mentioned, as they are still unknown to Harry at this point) just after midnight, and are soon driven off our path by what we can only assume is a werewolf, based upon its cries and glowing eyes. It pushes us into the Forbidden Forest, where Hagrid appears on Sirius Black's motorcycle to help us. We see centaurs and a frightened unicorn in the distance as we try to outrun the werewolf, zigzagging back and forth through the forest.
Suddenly the werewolf jumps out in front of us. Hagrid casts a spell that sends our carriage flying into the Shrieking Shack. The werewolf leaps after us and into the Shrieking Shack in front of us. Our carriage comes to a halt, dangling on the edge of the large boarded-up window we just broke through. Just before the beast reaches us, the grim (Padfoot) appears and fights off the werewolf. Hagrid casts a spell in the werewolf's direction, but it backfires, sending us flying backwards out of the Shrieking Shack again.
We lose Hagrid as we are catapulted towards the Black Lake. Suddenly the tentacles of the Giant Squid reach out and grab us. Buckbeak appears just in the nick of time, battling the squid and saving us from a watery grave. As we retreat to the castle gates beneath a rising sun, Hagrid rejoins us.
Just then, Dumbledore himself strolls by, sending someone to fetch Madame Pomfrey, explaining that Professor Lupinmay be in need of some help, and congratulating Hagrid on bringing us back to safety. "I couldna done it without the help of a couple o' escapees, if yeh know what I mean," Hagrid says, raising an eyebrow. "And to think they saved all those lives without the benefit of a timeturner," Dumbledore replies, winking as a hippogriff ridden by a shadowy figure flies away in the distance, with Fawkes calling farewell to them as he weaves in and out of the clouds.
The only way this could work is if either we're wearing goggles or the carriage itself is covered and has glazed windows or screens providing the effect of a moonlit night. Any way that is possible on a family-friendly but thrilling terrain-hugging roller coaster?