Unlock the superkit (the big AT-TE model outside Dexter's Diner) by filling the Jedi bar while playing. This incidentally unlocks access to the mystery door. Filling the Jedi bar is not a simple function of money, but also in grabbing all the pre-staged LEGO studs in each mission. Dying in a stage will reduce the Jedi bar (although once the bar is at maximum and pulsing, it cannot diminish).
In vehicular missions (particularly the pod race), the player needs to smash and destroy all the destructable objects to gain money. The invulnerability cheat ($1,000,000 in the shop) can help you get a full Jedi bar in most of the non-vehicular stages.
