Hidden Stages
Kongo Jungle (old):
Complete the 15 minute melee. This is hard. Get a beam sword and stand at the edge of the stage and use the a button to keep the wire frames away from you for the rest of the time. If you do this correctly, you should beat this easily.

Yoshi's Island (old):
Hit Sandbag at least 1300 feet in the Home-Run Contest.

Dreamland (old):
Complete target test with all 25 characters in under 2 minutes each.

Brinstar Depths:
Play 50 vs. matches

Fourside:
Play 100 vs. matches.

F-Zero Grand Prix: Big Blue:
Play 150 vs. matches.

Kanto Skies: Poké Floats:
Play 200 vs. matches.

Mushroom Kingdom 2:
Get a birdo or pidgit trophy

Superflat World: Flat Zone:
Comlete Classic Mode with Mr. Game & Watch, or complete event # 45.

Battlefield:
Beat all-star mode with any character.

Final Destination:
Complete all 51 events.