Location: The platform with the Zapfish on it will have a splat-switch. Inking it will reveal a launchpad to what Sheldon calls a "Deserted Island". Get away from the Octostamp DX to get the scroll, and the launchpad will take you back to the Zapfish.
Sardinium
Location:
After Checkpoint 5, climb over the giant moving wall. Look to your left and look down. There will be a small grassy platform with the Sardinium.
Armor
Platform Madhouse has 3 pieces of armor to collect.