Achieving 100% completion in the game isn't only about completing parts of the game. It's also about buying everything, including characters, extras, bricks and help items.
To unlock a mission named Anakin's Flight, collect thirty Gold Bricks throughout the game. Furthermore, if you complete this hidden mission, you'll get the Naboo Starfighter as a prize to use elsewhere in the game.
The following characters are unlockable in the game. Simply go to the Cantina, and access the codes command. Then, insert the following codes to unlock the corresponding character.
Additions by truegunner62 & michael82
If you go to the Bonus Room in-game and watch the new trailer for the upcoming Indiana Jones game, Indiana Jones will become a playable character that you can use. (unverified)
Lego City can be unlocked by collecting ten gold bricks throughout your journey.
New Town can be unlocked by collecting fifty gold bricks throughout your journey. Then, build in the "?" at Mos Eisley Cantina.
Below are a list of some skills. They can be unlocked by going to the Cantina and entering the codes listed.
Additions by michael82
To unlock Boba Fett's ship, Slave I, collect each Minikit Piece strewn throughout the levels. There are ten on each level that must be collected.
After collecting all 160 Gold Bricks in the game, you can build a "Stud Fountain" outside of Mos Eisely Cantina. This will give you an infinite supply of studs.
The following ships are unlockable in the game. Simply go to the Cantina, and access the codes command. Then, insert the following codes to unlock the corresponding ship.