Unlock Characters in Story Mode:
Beat the level and unlock these secret playable characters:
Medium level difficulty
2280 Return to Planet X: Ozor Mox
2019 NeoTokyo: Sadako
1853 Wild West: The Colonel
1972 Atom Smasher: Khallos
1920 Aztec Ruins: Stone Golem
1990 Siberia: Hybrid Mutant
1938 Chicago: Big Tony
???? Notre Dame: Jacque de la Morte
2280 Return to Planet X: Ozor Mox
2019 NeoTokyo: Sadako
???? Robot Factory: Machinist
???? TimePortal Station(?)

Retro Mini-Games:
While playing the game, equip yourself with the temporal uplink and then press B. Doing this will bring up a menu where you can play some retro games. The two I have are Anaconda (like the Snake game on celphones) and an Asteroids type game. You unlock the games by locating hidden cartrdiges throughout the game.

Unlock Cardboard Characters:
Beat the game on Easy difficulty to unlock cardboard characters.

Unlock Characters in Arcade Mode:
Earn a Gold medal in "Adios Amigos" (Arcade Mode) and unlock playable characters Hector Babasco and Lean Molly.

Unlock Rotating Heads Cheat:
Beat "Pane in the Neck" in Silver (Challenge Mode) to open up Rotating Heads cheat.

Unlock Streets Arcade Level:
Beat the game on Easy to unlock the Streets arcade level.