When the screen of Scooby and Scrappy Doo appears, simply type in "qwertyuiopasdfghjklzxcvbnm" in lower case to get continues up to Iceland.
On the title screen, type "STIG THE RAT" or "STIG THE RAT 2" to play as Stig the rat and for infinite lives.
Another source describes the cheat this way: Play once and die to go back to the title screen. Now type in "STIG THE RAT 2" or "STIG THE RAT" and begin play again to see Stig The Rat (which was apparently used before the Scooby sprites were approved) in Scrappy's place.