During game play, press pause, then press 1+3+6 to get 75 shop credits.
During game play, press pause, then press 6+7+8 to get rapid fire.
Finish all 10 Easy level waves. You should now have a new 'Insane' level on the options screen.
On title screen, press and hold pause+4+5+6 until screen fades to get invincibility. Explosion sound confirms. Press again to disable.