The Bonus Level is unlocked by getting True Jedi Status on all 16 Levels in the game. It can then be accessed by heading to the left side of the Cantina, and heading through the ? Door.
Episode ??
SANDBOX
An even longer time ago, in a galaxy far, far away....
It is a period of peace and tranquility. The galaxy sits in the throes of infancy as the CREATORS breathe life into the barren sand.
What will rise from the primordial sand? What surprises await the intrepid explorers? The face of creation can be changed...
Playable Characters: Salacious Crumb, Slave Leia
Enemies: ?
True Jedi at 10,100: Here for some reason? Not worth it. I don't even know if it is possible. There's also no Minikits here despite what the Cantina beforehand says.
Um... this is a thing? This is a large open sandbox with stuff in it. That's it. I don't even know what you're supposed to do here. It just... is... I guess. I mean, there's an Exogorth. Some Sail Barge type things in the sky I don't even think you can get to.
Anyway, Walkthrough. Press the Pause Button, go to Mos Eisley Cantina, and head back there. I'm serious. Apparently this registers the stage as complete so... yeah. This game is a broken mess.