Debug Menu
At the title screen, press and hold A & B on the 2nd controller and Select on the 1st Controller, then press Start. If done right, the debug menu will appear.
Extra Money
Whenever you see the intro screen to a stage, the screen declaring the stage number, hold Select and then rapidly and alternatingly press Left and Right (or Up and Down) on the D-Pad. If you press them fast enough (usually eight times per second), every 8th press will net you an extra 1,000 money; you can easily rack up to 4,000 money per stage screen this way.
Level Select
At the title screen hold A + B buttons on the second controller, press on the first controller A, A, A, A, B, B, B, B, A, B, A, B, A, B, A, B.