During the game play press [Esc] to display the pause menu. Then, press [~] to display the console command prompt, which will be a bright line with an input prompt below the menu. Enter one of the following codes to activate the corresponding cheat function for the current level. Two things tho' You will not be able to see what you are typing and you must re-enter the cheats for each level as they get reset.
viewself - Set camera to behind view
freecamera <true or false> - Camera and pawn not rotated together when "True"
invisible <true or false> - Toggle invisibility
avatar <class name> - Possess a pawn of indicated class
summon <class name> - Summon requested class
killpawns - Kill all pawns and enemies
god - God mode
allweapons - All weapons
allammo - Maximum ammunition
loaded - All weapons and ammunition
fly - Flight mode
walk - Disable flight and no clipping
ghost - No clipping
amphibious - Stay underwater without damage
teleport - Teleport to area currently looked at
changesize <#> - Scale player size to #
changesize * - Restore default player size
setspeed <#> - Set player speed to #
setjumpz <#> - Set maximum jump height to #
setgravity <#> - Set gravity to #
slomo n - Set slow motion factor
Start the level where you have to go to the log mill to kill the "Twins". Enter one of the rooms in the three gin distilleries at the start of the level that has a "table" constructed from a wood plank laying across barrels. Use dynamite to blows up one of the walls in that room to find a hidden area. Inside is a glowing purple Runestone.
In the level where the boss is playing an old phonograph, find the three hidden watermelons. They are found in the following locations: just before going down to the wall of adobe buildings on the left; hidden in a deep gully; and on top of the mountains. Shoot all three of them to play a shooting gallery mini-game.