Galactic Enforcer color set:
Press Up, Right, Down, Left, A, B, Select at the level selection screen. A sound will confirm correct code entry. Go into any level and you will be in the "Galactic Enforcer" color set. Both Ben and all his alien forms will be black and white and official "Galactic Enforcers".

Gwen 10 skin:
Press Left, Right, Left, Right, L, R, Select at the level selection screen. A sound will confirm correct code entry.

Gold color:
Press A, B, X, Y, L, R, Select at the level selection screen. A sound will confirm correct code entry. Go into any level and you will be in a gold suit.

Upchuck:
Press A, Left, Y, Right, X, Up, B, Down, Select at the level selection screen. A sound will confirm correct code entry. Upchuck will now be permanently unlocked; just select his silhouette on the Omnitrix.
Agent Spectre