At the 'Main' screen select the 'Password' option and enter the code !V?Y3T?8 to unlock ALL these features in the game without having to play through it.

Level Unlockables
At the Password screen enter the following codes to unlock the corresponding level

Unlock Level 1.2:
Enter the code CBBCBBBB

Unlock Level 1.3:
Enter the code BDBFBBBC

Unlock Level 1.4:
Enter the code CDBFBBBD

Unlock Level 2.1:
Enter the code LBCKBBBF

Unlock Level 2.2:
Enter the code MBC3BVBC

Unlock Level 2.3:
Enter the code LDGYCVBC

Unlock Level 2.4:
Enter the code MDGYFVBB

Unlock Level 3.1:
Enter the code LBB?TVBB

Unlock Level 3.2:
Enter the code MBB7KVBL

Unlock Level 3.3:
Enter the code 4DB7KVBL

Unlock Level 3.4:
Enter the code SDB7KCBL

Unlock Level 4.1:
Enter the code 4BG7KCBL

Unlock Level 4.2:
Enter the code SBG7KCBL

Unlock Level 4.3:
Enter the code 4DG7KMBL

Unlock Level 4.4:
Enter the code SDG7KMB4

Unlock Level 5.1:
Enter the code 4BBY3MC4

Unlock Level 5.2:
Enter the code SBBY3MF4

Unlock Level 5.3:
Enter the code 4DBY3MK4

Unlock Level 5.4
Enter the code SDBY3MT4

Unlock Final Level: Scooby versus Masked Guy
Enter the code 5DBY3MT8


Unlock Alternate Ending:
Beat the game and when prompted for a code enter SD2