Okay so the cute plant is the thing that replaces the house cup so you dont die. Once you have the house cup you want to visit the Great Hall and there will be a celebration.
The fish and yarn are not for anything. They were just an added thing I put in there for fun just like the Twilight book.
The snacks are for petting the hippogriff successfully otherwise you'll be harmed and possibly sent to the Hospital Wing. As for why are there two different place where you can pick up a snack item-- just because. lol
Thanks for playing! I hope you had fun!