To unlock the Glowing Skeleton costume, reset your costume and shock yourself at an electric trap. Pause the game while getting shocked. If done correctly, you should see a skeleton show through your character. Hold the PlayStation Logo to turn off your controller. Then, turn the controller on again when prompted to get a costume with a glowing skeleton.
In Story mode, start a mini-game. After the entrance is gone, pause the game. Hold down the Playstation Button and select "Turn Off Controller". When the screen says "Press PS button to play!", press the PS button and unpause the game. You will have lost 0 lives and appear on the scoreboard.
Most levels have multi-player prizes area that requires two players to unlock. However, most of these areas can be easily completed with one person and two controllers. Start the level in single-player mode. Make your way to the multi-player zone. Stand near the closest checkpoint and press PlayStation Logo. A second Sackboy will spawn from the checkpoint. Have him stand on the switch or hold something down while you collect the prizes, then hold PlayStation Logo on controller two for a couple seconds until the option "Turn off controller" appears. Select it, and after a few seconds the second Sackboy will disappear. You can now finish the level in single-player mode.
To unlock the Burnt costume, reset your costume and then go to where there is a fire and burn yourself. While you are getting burned, pause the game. You will see a charred character. Turn off your controller by holding the Playstation logo. Then, turn the controller back on when prompted to unlock it.
Choose Dark Matter in the level creator and set it to the smallest size. Create a line and it will be almost invisible, standing in the air. Drop from Fly mode and, if you land on it, you will "float".
Note: A controller with a turbo feature is required. If you have the Paintinator power-up, pause game play in Create mode. Enable the turbo to shoot directly at the ground or run while shooting to elevate.