Unfortunately, I think the only thing in the Harry Potter canon (books and movies) that's really suited to being a roller coaster is Gringotts. So... that's done. The next obvious thing I can think of is something quidditch themed, since the experience of zooming around on a broomstick is probably close to that of riding a coaster. Of course, you can't make a roller coaster where the ride vehicle is a broomstick, and you'd see the track which would destroy even the pretense of illusion.
So there's the flying Ford Anglia, the Thestral carriages, the flying Motorcycle & sidecar, or... some enchanted flying benches? They're going to have to make something up, even if it's as simple/silly as "Hagrid enchanted his motorbike to give you a tour through the forest," or "Luna managed to enlarge the Ford Anglia".
A roller coaster is a weird fit for a Forbidden Forest ride.