On some (but not all) levels, look for gates of a different color other than the normal blue gates. These gates are harder to get through, but they earn you extra points and warp you ahead.
To get easy play points just start a new game, beat it all the way until the water levels (unless you're a real hot shot) then delete the new game and repeat.
To unlock extra levels, complete the following challenges for the corresponding levels in Challenge Mode:
Complete all levels in Worlds 5-10, plus the Extra Expert levels (all without continuing) to unlock Master Difficulty. Crikey! That's tough.
You must get all all of the items in Gift menu.
You must get all of the mini-games first. This will trigger new music playing during the main menu.