Cheat Code
Enter "WE5T ONE" at the Retro Password entry screen after selecting "New Game" to start as Hu-man with all hearts, full inventory, and 900k gold.
Easy Gold and Items
If you want fast gold and items early on in the game, you can go to the place in the jungle area shown in the screenshots provided below after becoming the mouse-man. Kill the skeleton here, then go left one screen and kill the skeleton here, then go back to the right and kill the skeleton here again. Rinse and repeat until you have all of the gold and items you want. As far as I can tell, they will usually drop anywhere between 20 - 70 Gold and, slightly less frequently, an item (whirlwind, arrow, or boomerang), every time. Now, they are difficult to kill with the mouse-man's normal attack because it is short and they begin jumping as soon as they get close. So use the boomerang item for a quick, ranged kill. As long as you always catch the boomerang as it comes back, you will never use up the item. I gathered up about 4000 Gold in only about 10-15 minutes.
I've since discovered it is faster if you only kill the skeleton on the left screen (with the bat), then leave right, come back immediately and kill him again (repeat). He is close enough that you can fire off your boomerang as soon as you re-enter.
I found an even faster way. In the same area as the skeleton and the bat, go all the way left. At the end, you'll see a fire-spewing plant and a skeleton next to each other just before continuing and exiiting left. Leave left, go back, then stop (to avoid the fire projectiles) and fire off two boomerangs (because the plant takes three hits). Both tend to drop the same amount of gold and items (for some reason, this skeleton started dropping a lot of fire items and a few lightnngs). Then just exit left, go back to the right and repeat.
Extreme Difficulty (v1.0.3c)
When choosing a difficulty, hold down X for a while and eventually you'll be given the option to play the game in Extreme difficulty. This mode will only appear if the game has been updated to v1.0.3c or later.
Human Body Swap
After beating the game or using a password to re-obtain the human form you start the game with, go to any transformation room while in Hawk-Man form and hold Up+X+B while you fall on the platform. When you transform you will turn into the character you didn't choose at the title screen (Hu-Girl if Hu-Man and vice versa). The effect will save to your file, essentially acting as if you'd picked that character from the beginning. The trick can be repeated whenever you want to swap back and forth between the characters.
Modern/Retro Split Screen
Rapidly press the ZR button to switch between Modern and Retro graphics. As the horizontal wipe effect moves back and forth, it'll gradually slow down and eventually freeze in place, leaving the left of the screen in Modern mode and the right of the screen in Retro mode. How much of each side is present is dependent on the timing of the button presses and can be manipulated with practice.
Cheat Code...