Bypass introductory sequence
Type PP during the monitor adjustment screen that shows a skull. Then Right-click at the title screen to bypass the introductory sequence.

Advance in game
Go to the skull load-in screen. Then type this case-sensitive text, with spaces: Roberto Dan R Zaphod B [Space].

After you type Roberto, Stauf will say, " Duh.... that's not it Karl!". Ignore him, he is trying to trick you. After the B, type [Space], then any of the letters below to reach that point in the game:

A: End of disc one.
S: End of disc two.
D: End of disc three.
F: End of disc four.
E: The "Olive In Stein" riddle, good for using the Kellyn code.
G: The very end, at the "Let's Make A Real Deal" intermission sequence.

Hidden video
Go to the television In Dutton's room at a point where you can see the close-up view. Type Kellyn to look at a hidden video.

Mocking cheaters
In The 7th Guest, typing Zaphod Beeblebrox will give you open house access. Doing this in The 11th Hour will only result in Stuaf to mock you.

Myst reference
Reach the point in the game where you have access to the altar room. On the side here, is a bowl you can click. There is a page sticking out among the bones that says "Missed". This is an obvious reference to Myst and pages.

Ending bonus
Finish the game to access the "Open House" file. Now load it to be able to play any puzzle and a unlimited number of times.

Kaleidoscope screensaver
Start the game and go anyplace in the mansion. Stay idle and in time the colors on the screen at that time will blend together as a screensaver. It is different from each viewpoint in the mansion.