A hidden credits screen can be accessed by holding 0 on either controller while on the title screen.
Contributed by MeleeWaluigi
A birth announcement hidden in the game's ROM can be accessed by holding 1 on the left controller and 4 on the right controller, then pressing reset on the console. The screen announces the birth of game designer Steve Ettinger's daughter, Jessica Louise Ettinger:

Jessica Louise
July 19, 1988
3:25 a.m.
Welcome To The
Fun And Games!
Way To Go, Linda!
Contributed by DeerBoarDude