Pitfall: The Mayan Adventure Cheats

In Level 4 (Ruins) you will see an Scorpien from the Original Pitfall on a platform with a door. Use the idol's tounges to get up to higher ground and jump down onto the platform. Go in the door, run to the left hand wall and you will hit a hidden spring. You will go flying through a fake wall. Run to the right and down the portal.

Now you are playing the original Pitfall. Enjoy the wonders of the Atari 2600 (or whatever the number is)

(You end up at the door when you run out of lives in the original.)