What you are referring to are known as "Easter Eggs". Here the ones that I know of:
**** 2004 (and LT/Desktops)
****
On the Format pulldown menu, select color. Activate the Index Color tab (ACI tab) within the Color Dialog Box. Hold down the control key and double click (left mouse button) over color
152 ("AutoDesk Blue"). A new dialog box opens. Make sure your speakers are on. Use your left mouse button to Orbit and the right mouse button to Zoom.
**** 2000 ****
**NOTE: If you changed the Start in Directory the egg will not work. To see the egg, go to the AutoCAD folder and double click on the ACAD.exe file.
**
On the Format pulldown menu, select Style. Hold down the shift key and left click on the dimension preview image. A new dialog box opens displaying a large T (for Tahoe). Use the Play pulldown to start the animation.
**** R14
****
On the Help pulldown menu, select About AutoCAD R14 Hold down the shift and control keys, left click on the AutoCAD R14 image on the left side. A new dialog box will display a picture and scrolling text. If you left click the scrolling text will speed up. If you right click the scrolling text will slow down.
**** Bonus Tools for R14
****
On the Bonus Tools pulldown menu, select About Bonus. In the center of the letter O, of the word Bonus, you'll find a red dot. Left click on the dot.
**** AutoDesk Express Viewer
****
On the Help pulldown menu, goto the Help menubar and select About. Hold down the control key and left click on the image. This will launch a browser window and take you to a page titled "Larry's Head"