Please bear in mind that I hadn't actually SEEN Yu Yu Hakusho when I had this dream. Not that it would have made sense if I had.
It all started with Hiei on the way to school. As he was walking through the woods, a mean old bully started picking on him. Apparently he was mad because Hiei was responsible for wrecking his truck. Hiei was scared of the bully, so he went running into the school, searching for a place to hide. Eventually he found a bathroom, but it wouldn't lock! The bully would find him at any moment! So Hiei did the only thing he could do: he jumped into the toilet and fell down into the sewer.
Luckily for Hiei, the sewer was apparently full of mud. He wandered around down there for a while, where he met Gateau from Sorcerer Hunters. I can't remember what they talked about, though. = / Eventually the sewer opened up into a huge lake, in the middle of which was a perfectly round island. The lake and island were brightly colored and looked like something out of Mario 64. Hiei figured the only way across the lake was to swim, and so he did. When he got halfway across he pulled himself onto the island to take a rest. Unfortunately, Vegeta was also on the island, running in circles around the perimeter, and when he saw Hiei he was really upset.
"HOW DARE YOU TRESPASS ON MY ISLAND!" Vegeta screamed, before turning into a bull and butting Hiei off. He flew through the air and landed with a splash - luckily Vegeta had pushed him in the direction he'd wanted to go.
After Hiei finally made it to the beach, Kurama appeared and told him that he should get on the plane that was a bit farther down. Hiei did as he was told, and the plane ended up dropping him back at school, where my marching band saxaphone section was dropping pennies in the toilets, which somehow made them erupt into fountains of water.
The end... almost! At this point it turned out I had been watching the whole thing in a movie theater, and when it was over I decided I had liked it so much that I headed right back in to watch it again. (There was a trailer for Home Alone on before it.)