Secrets: Just like the good old days.
Play as Luigi in Single-Player
Hold down L and R when selecting a save file.
Fireworks, Flagpoles and Mushroom Houses
Jumping "over" the flagpole (hitting its highest point) will net you a 1up.
To get fireworks and the original Super Mario Bros. theme at the end of a level, jump onto the flagpole when the last two digits of the timer read 11, 22, 33, 44, 55, 66, 77, 88 or 99.
In addition to fireworks, jumping on the flagpole when the last two digits of the timer read 11, 22, 33, 44, 55, 66, 77, 88 or 99 will make Mushroom Houses appear on the first space of the world:
Red Mushroom House: Last two digits of the timer read 11, 22, 33
Green Mushroom House: Last two digits of the timer read 44, 55, 66
Mega Mushroom House: Last two digits of the timer read 77, 88, 99
Secret Worlds
Unlock World 4: Beat the boss of World 2 as Mini Mario
Unlock World 7: Beat the boss of World 5 as Mini Mario