Below is a page of drawings from one of Eric Harris's notebooks (taken from the Columbine Report). The sketches look to be similar to the stuff you see in Doom games, though the illustrations on the back are ideas for bombs and weapons that have nothing to do with the game.
Click on the scaled-down images below to see the full-sized scans.
The front of Eric's journal page
The back side of the page, enhanced to show the sketches of bombs
Clearer image of the back side
Close up of the bomb on the other side of the page