Level select:
Press Up(2), Down, Up, Down, Up(4) at the title screen.

Display level statistics:
Press Select at the title screen to display the time completed and high score for all levels on all saved games.

Hint: Item activation:
When you activate an item, after you put it where you want it repeatedly press A and you can activate another one. Do not do this too frequently, or the game will not let you stop activating items.