Instructions
Switch to Hagrid so that you can use his spells and abilities in Diagon Alley, as Harry does not have any spells at this point in the game.
Cast the Leviosa spell on the bricks in the alley way that is outside The Leaky Cauldron to enter Diagon Alley.
Head to the far end of the street to find a barrier blocking your path. Collect the red spider to the left and the yellow flowers to the right, both of which are ingredients for the potion that you will make.
Cast the Leviosa spell on the ice cream cart nearby, lifting it off the ground and making it drop a blue Popsicle.
Collect the blue Popsicle and place all three items in the cauldron next to the barrier. This will cause the cauldron, along with the barrier, to explode. Head through it to get to Gringott's bank.