Oh, here's one! Go into World 1-3, then progress until you see this platform:
Get onto it somehow, then hold down for 6 in-game seconds. Then, you should fall behind the scenery. Go to the end of 1-3, then keep moving right. You should get this:
sorry to disapoint you but i doubt anybody would post. if you want those secrets just check online... its a very old game, i doubt anybody knows secrets that are unknown.