Super Mario Bros. / Duck Hunt / World Class Track Meet Cheats

Hold A and press Start at the Gameover screen to continue from the first level of that world.

When playing on Mode A (the one with only one duck), controller one can control the ducks to make a sort of multiplayer game.

Start a target shooting game and have an extra controller plugged in to port 1 and the light gun in port 2. Press start on the controller in port 1 to pause the game when you unpause the game the targets will remain frozen for a few seconds giving you a clean shot.