Obtain all 100 wizard cards (you have to start a new game if you missed some in the Burrow, you also have to have linked up with a completed GBA game to unlock Gringotts) then speak to Dumbledore. He will give you the 101st card. Once you have received this card he says different things as well.
Once you've collected your Standard Book of Spells (Grade 2) in Diagon Alley, you can cast Flipendo to break barrels and uncover Sickles that you can use to buy items such as Stink Pellets at Gambol and Japes.