Enter the "Extras" menu, then select "Codes". Enter the one following codes to activate the corresponding cheat function.

Daphne Karate Black Belt costume
Enter 2839 as a code.

Fred Knight costume
Enter 4826 as a code.

Scooby-Doo Egyptian costume
Enter 1585 as a code.

Shaggy Native American costume
Enter 3726 as a code.

Velma Pirate costume
Enter 6588 as a code.