Hmmm only thing that comes to mind is a shrinking potion (They were tested in their first year with this potion)
I found quite a few websites that are canon with the book that have a list of potions and sometimes give a recipe if described so in the book. If you want a link to it just ask and ill post it
Setting Off Fireworks in Potions Class
Thank you for all of your suggestions. Coming up with a simple potion is not all that important to the story I am writing, but I like to describe things in detail.
Tim the Enchanter