To unlock Han Solo as a playable character, you must find five Star Wars-related dolls strewn throughout the game. Below are the five dolls and their general locations.

C-3PO - Raiders of the Lost Ark, The Lost Temple
Chewbacca - Raiders of the Lost Ark, City of Danger
Leia - Temple of Doom, Free the Slaves
Luke - Raiders of the Lost Ark, Into the Mountians
R2-D2 - Last Crusade, Desert Ambush