"Batteries! That's the answer!"
Be sure to pick up the Sense Materia when you arrive in the park.
Grab a Turbo Ether from the northern house where you DIDN'T take the 5 Gil from the hidden drawer earlier. Not missable, and not the most amazing item in the world, but not too shabby at this point in the game!
After speaking to Aeris's Mom and returning to Wall Market. You'll need to get three Batteries from a man in the Weapon Shop. You have to use two of the Batteries to proceed. Don't use the final Battery as it only gets you an Ether of which there are plenty of in the game. The Battery is useless but it's cool to keep it in your inventory.