Go back to the Gold Saucer (you can reach it quickly with the Tiny Bronco by following the river through the center of the continent) and go to the battle arena. Inside Dio's Showroom, you'll find the keystone. Dio agrees to let you have it if you fight in the arena. If you win all 8 battles, you'll get a Protect Vest and a Choco Feather but even if you lose you'll still get the Keystone.
Go back to the entrance. The tram is broken so you're stuck here for the night so Cait Sith gets you into the Ghost Hotel for free. During the night one of the girls will show up and force Cloud to take her out. First you take part in a play, then ride on the gondola. After the ride, you'll see Cait Sith stealing the Keystone. Chase him until he leads you to the Chocobo Racing, but before you can catch him he hands it over to Tseng. Back in your room, check the cabinet for an Elixir.
|Notes:||Which one of the girls shows up in Cloud's room depends on whether you've been nice to Aeris or Tifa. If you were mean to both, then it's Yuffie!|