Get builtin cheats:
First try to find the default.cfg file.
Go to GameData/indy/default.cfg and edit the file with texeditor.
Note: Always make a backup file, before edit.

Modify the following:
pack:0
title:0
freescenedata:1
vsyncinterval:2
milessound:1
nocontrol:1
cheats:1
resmgrload:res.dat
fullscreen:1

Save your default.cfg file, then run the game.

You will have access to the builtin cheatmode that provides:
Unlimited Health
Unlimited Oxigen
God Mode        
Unlimited Ammo  


Disable AI:
Note: This procedure involves editing a game file; create a backup copy of 
the file before proceeding. Use a text editor to edit the "vars.cfg" file in
the "\lucasarts\indiana jones and the emperor's tomb" folder. 
Locate the "aiDisabled" line and change it to "aiDisabled=false". 


Art gallery:
Collect all artifacts in the game to unlock an art gallery.