Inside the game manual, in the bit explaining the password screen usage, is a screenshot with a password in it. This password takes you to the Volcano stage of the game as Grant.
The password is: AVVUP8EL
Select Grant and the easy difficulty on the options screen. Use the controller configuration option to return to the default settings. Then hold A and press Start and the additional options menu will appear.
Enter NYUKNYUK as a password.
(This is a reference to the 3 Stooges, by the way.)
Enabling this code allows you to use player 2's controller to move (fly) Grant or the Raptor around the screen. A, B, and C have various effects, like restoring health and restoring all items.