Level select - Hold the Command + Option buttons and select "Begin New Game". Then, type the level number and select. This works on all Marathon maps, and Marathon 2 and Marathon Infinity as well.

Marathon logo Gamerpic - Accomplish the "11th Clan of the S'pht" achievement to unlock the Marathon logo Gamerpic.

Vidmaster Gamerpic - Accomplish the the "VidMaster" achievement to unlock the Vidmaster Gamerpic.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

SEVEN (7 points): Get seven fist kills in the game.
Bob-B-Q (14 points): They're Everywhere!
11th Clan of the S'pht (27 points): Venture into the Citadel and learn the secrets of the S'pht.
Pfhor the Win (42 points): Complete the Single player game in any difficulty.
Tastes Like Chicken (10 points): Get 20 Tozt-7 kills in a Multiplayer game.
Pfhor Score and Seven Years Ago (10 points): Win a Multiplayer game.
I'm Invincible! (10 points): Kill an invincible opponent with the Fusion Pistol in a Multiplayer game.
Make Someone Pay (10 points): Get 20 Rocket Launcher kills in a Multiplayer game.
Bigger Guns Nearby (14 points): Find the Assault Rifle in a Single player or Co-Op Game, it will let you kill more Pfhor.
Hats off to 819 (21 points): The secret of 2401 lies within.
VidMaster (35 points): I give no secrets.
World Domination (0 points): You want to join the 7th Column?