Quattro Adventure

Go underwater (Right of the start) and find the platform on the left side of the sunken ship. Jump onto the platform and move to the right side. Pressing B will allow you to view the credits.

Level warps are certain spots in certain levels that will send you directly to the next level without the need to collect any more boomerangs or reach the regular level exit. You will not receive any points for unobtained boomerangs or time left on the clock if you use them, however.

Shark City: Climb up and jump above the hill on the left.
Shark Fin Creek: Just above the first shark pool; it's easiest to jump onto the center of the crumbling platform and fall straight in.
Tower of the Forgotten Knights: Climb to the top floor, then jump off the far left edge.
The Secret Chamber: At the start, carefully jump over the chair to your left, then move further left and jump again if you haven't already hit the warp.
Backstab Alley: Jump onto the very first platform in front of you at the start, then jump up to the left.
Hopping Mad Halt: On the bottom floor, in between the center and right edge of the screen, do a small jump.