4. Unlocking World Four
There's a pretty fair chance that even if you beat the game, you may still be looking at the World Select screen thinking, "How the hell do I unlock those two locations?" Well, the answer's pretty simple - you gotta be small to sneak in.
You can either grab the mini mushroom from 1-4 or head to the final castle in world two and nab it there, but either way, you have to beat the desert boss as mini Mario. Normal stomps won't affect the boss, so you have to use the butt stomp three times to win - but watch out, it only takes one hit to die as the tiny Mario.
5. Unlocking World Seven
As you may have guessed, getting to world seven is done the exact same way - beating the boss as mini Mario. Once again, you could grab the mushroom from 1-4, but there's a chance to grab one just before the final exit to the giant plant boss. The item block in question is constantly cycling through several power-ups, so you have to time your jump to eject the mini mushroom - it'll take a few tries, but it's easier than running through the whole level as mini Mario. Topple the boss by butt-stomping his stomach (in the least offensive way, we suppose) and you're off to world seven, ready to start searching for every last secret the game has in store.