Voodoo Vince Easter Egg - Literally an Easter Egg.

Select the level "Inside the Cosmobot."
This is the final level, but this shouldn't deter you, as playing this far is a blast.
Anyway, jump on the girders in the beginning of the level, starting from the stool. When jumping on the girders, you should jump on the blue lights above them as it will make the jumps easier.
Make it past the gear obstacles and you will arrive at a cannon. Press X to hop in. You can't aim the cannon, but you will notice two giant swinging tires ahead of you. Wait for the first of these two tires to cross over the cannon's aiming reticule. Aim for the side of the tire, not through it.
This may take a few tries. If you manage to hit the thick part of the tire, move Vince over to the right in midair just as you hit it, and you will fall into the center of the tire. Inside the tire is an Easter egg, like the ones kids open on Easter.

You can also reach the egg by simply jumping onto the cannon and using Vince's spin jump. If you mess up, just hover back to the cannon.