Any CAD software often makes its own April fools jokes but if you insist on adding your own jokes to AutoCAD's own AI, here are some tips that will please your colleagues for sure.
You can e.g. let AutoCAD display a seriously looking "system" message on any opened drawing:
For this, you just need to create a text file ACADDOC.LSP in the AutoCAD folder with the following content:
(alert (strcat "AutoCAD detected a severe license violation for the user \"" (getvar "LOGINNAME") "\"!\n\nInformation about the license and your computer were sent to Autodesk and BSA.\n\nThe unauthorized license will be now terminated and wiped-out from disk."))
A very rich range of options appropriate for today is offered by the mechanism of undefining and redefining of AutoCAD commands. E.g. by entering UNDEFINE LINE you can unlearn AutoCAD drawing lines - will be surely appreciated by any colleague. Again, you can use the automatic mechanism of ACADDOC.LSP - content:
(command "_UNDEFINE" "_LINE").
You can even spice this up by defining a new, custom command "LINE", which might then draw e.g. circles:
(defun C:LINE ()
An effective trick is also defining some "normal" key, like "A", as a hotkey - keyboard shortcut (see CUI). Any attempt to type "A" in AutoCAD command line will then run the command or perform an empty action - e.g. *Cancel*.
Colleagues with unbearably large monitors can be relieved by showing all the necessary tools at once: -TOOLBAR ALL Show
If your collegue has a PC equipped with speakers, you can surprise him or her by a rock song launched automatically from a podcast or a music clip on Youtube. You can start any web page (or document) using the command (again in ACADDOC.LSP) - e.g.:
(command "_BROWSER" "http://www.youtube.com/watch?v=-tJYN-eG1zk")
It is interesting that some popular AutoCAD April jokes are no longer working in newer versions of AutoCAD - Autodesk checks or limits the settings now - you can e.g. no longer set an extra large PICKBOX or set the cursor color to the background color (a very pleasant combination visually).
Update: My friend Jan from Germany reminded me about our older "talking" LISP utility "SPEAK". With it you can surprise your collegues with the Space Odyssey classic:
SAY Do you want to run AutoCAD? I'm sorry Dave. I'm afraid I can't do that.
PS: good advice - stay close after you deploy such jokes - to be able to revert them to normal as your employer may have different opinion on the CAD activities you perform...
Have a nice April 1st and ... don't get caught!