when i was a kid i unlocked the swimsuit by playing the game normally.
i don't think i beat the game in under an hour, but I did think I did beat it twice (beating it takes you back to the beginning).
funny nobody seems to be mentioning this... i didn't have access to nintendo power or any other cheat resources, so yeah, pretty supprising to find out my character was a girl at the end :P
(upon further research, apparently that honor goes to Ms. Pacman)
It blew my mind when I was about 8 years old, in a good way though. I still find it an interesting choice to leave the gender reveal to the end, playing upon our (or mine) expectations and trivializing gender all along the path to defeating Mother Brain!
http://www.vgmuseum.com/end/nes/b/met.htm