Outside of The Leaky Cauldron is Diagon Alley. In Diagon Alley, you can head to other stores to obtain items and supplies. At Madam Malkin's Robes, you can spend all of your hard-earned studs (the currency) to buy new characters. At Eeylops Eye Emporium, you can head upstairs and buy Red Bricks which contain cheat codes of sorts to use. And at Wiseacre's Wizarding Supplies, you can buy fun new spells to cast on anybody you like. Casting spells is an important part of the gameplay and is accessed by a spell wheel in which you can choose many ranging from Wingardium Leviosa to the dreaded Aveda Kedavra.
LEGO Harry Potter adds to the replay value by having a bunch of collectibles for you to obtain. If you want to get 100% completion, you're going to have to do things such as find and purchase all 167 characters, rescue 50 students in peril, find 200 gold bricks, and much more. Hogwarts school is open to you at all time where you can search the many rooms and floors for all these collectibles. You will have to wait though if you try to get everything early because many characters have their own special abilities pertaining to them.