Credits Tablet
When you defeat the final boss and speak to Cap'n Cuttlefish, you will obtain a stone found in the Octo Valley that allows you to view the credits and play the extra paint mini game.
Hero Suit Replica

Beat the final boss in Single Player Campaign to unlock the Hero Suit Replica set for use in multiplayer - it will contain a hat, shirt and shoes.
Octoling Suit Replica

Beat the final boss in Single Player Campaign to unlock the Octoling Suit Replica set for use in multiplayer - it will contain a hat, shirt and shoes.
Squid Racer Speed Boost

Before the level starts and during the countdown, press accelerate right before the '2' changes to '1'. If you do it correctly, you'll get an extra boost.