Here are a list of different items you might find around Hogwarts.

Chocolate Frogs	These are a real life savers, as they will replenish health. Found mostly in chests and can be bought at Fred and George's shop for 10 beans.
Bertie Bott's beans	You can find beans everywhere in the castle. These are used for buying items from fellow classmates. You can also get goodies from Fred and George's shop, such as wizard cards and chocolate frogs
Wizard Cards	There are a total of 80 wizard cards hidden around the game. They are in sets and each set is in a specific level. Collect them all to complete the game!
Cauldron Cakes	Perhaps the most useful of all items, as they are the main items exchanged in Fred and George's shop for wizard cards and portrait passwords
Pumpkin Pasties	Also another important item to collect for some wizard cards.
Portraits	Found throughout Hogwarts, these passwords are useful in finding many goodies hidden. Some passwords open portraits that allow one to go from one floor to the next, and others can be used to find secret areas. All found at Fred and George's Shop