Alien Disruptor:

Beat the game on Expert to get the disruptor.

Debug mode:

Hold Z and press Right, Left, Up, Down, A, B, Start when "Press Start" appears on the opening screen to access a debug menu.

Eighth Level:

The secret eighth level is accessed by finishing the previous seven levels on the "Hard" setting.

Shoot down the President:

Hold Z, C-up, and C-down during game play, then fire a homing cluster at an enemy plane. The president will jump out if the H-Cluster hits.