Mr. Mullpepper's Apothecary:
On the left when you enter the store are some book cases. Use Flipendo on the vases on top of the book cases and when they fall they will reveal chocolate frogs which you can collect. Avoid the vases when they fall or you will lose some beans.
The Whomping Willow:
Stay as far from the branches of the the Whomping Willow as you can when you are trying to save Ron. Hit the branch when it has a green light then the eye of the tree. Avoid the boulders the left branch throws.