Secrets can be found in the game mainly after obtaining the broom swap power up.
They are shown in the menu under the ??? item.
“Sean From the entrance of the Debug World, go RIGHT, RIGHT, RIGHT, DOWN, DOWN, LEFT, DOWN. You...Secret 7
“A can of SPAM. In the fields near the north entrance, use swap to go left where rocks block your...Secret 2
“Blue Square From the beach warp, go south five screens(by using the walkable offscreen tiles...Secret 12