A full map of the game and the secret area is available here. The rar file called Full Map.rar is not recommended, as it contains a BMP file which expands to a ludicrious 160MB. The PNG files are mirrored in another RAR file here.
According to chironex in this forum topic, you can make the following changes to the settings.ini file accompanying the game to unlock secret extra stuff:
Here are changes you can make in settings.ini to change the Knytt physics: 1: The hang glider [Secrets] Extra=Hang Glider Let you glide through the air. Jumping and moving are harder. 2: The balloon [Secrets] Extra=Balloon Greatly reduces gravity, but makes it harder to accelerate. 3: Physics mods [Physics] Max X Velocity=0 Max Y Velocity=0 X Acceleration=0 X Deacceleration=0 Gravity=0 Jump strenght=0 Jump hold strenght=0 Lets you change physical constants in the game. Replacing 0 with any other amount adds that to the normal value of the constant. Note the misspelling "strenght". Yes, you must still spell that incorrectly in the same way to get the value to work.
In addition to the low gravity (found in the bottom-left corner) and mega run (found under the left half of plateau-spire in the desert) cheat codes, there are two secrets found in-game. The first is located in the bottom-most row of the game, to the right of the lava cavern and left of the sand column. The second is located in the top-left corner of the world map, in the cloudy area.
Deceased Crab has a "Let's Play" of the game on YouTube: