The secret level
in Toejam & Earl
Go back to the level 1 once you get a present with either the Rocket Skates, the Icarus Wings, or the Innertube. Once there, make your way to the very southwest of the map. (If you want or even care, there's an island full of gifts on the diametrally opposing point of the stage.)
You'll get to an island with a hole in its middle... just fall on it.
You'll fall to level 0! It's simply a grassy rock floating in sheer space. (Probably Astral Turf.) The only things that exist in this level are a bathtub that'll recover all your health if you dive on it, and a lemonade stand that'll give you a free life! (Only one time, folks.) Just jump off the rock to return to the highest level you've been to.