Get seven Resse's Pieces and give them to Elliot. Find a flower in the bottom of one of the wells and make it bloom. Complete the game, press the fire buton to play another game, do it again (get seve, give to Elliot, bloom flower_ and the flower turns into one of the other characters from the E.T. game. Do it a third time and you will get the designer's initials when the flower blooms.
Purposely allow E.T. to die on the landing pad while waiting for the mother ship. The game will crash if Elliot arrives on the screen but doesn't save E.T.