Hidden Pictures and Themes
Go to the data folder and open the sound folder. In it you should find hidden pictures of Kurt, one of the creators. In the theme folder you will find different themes for your PC.

Level Editor
At the main menu hold Ctrl and push E 6 times for three options: Make your Own Level, Edit a Level, and Quit.

Campaign Modes
At the Player Select Screen, Push the letter "C" five times. You will then get to chose one of three campaign modes.