Tiny blue cubes are hidden throughout the Star Wars Play Sets, collecting them unlocks Concept Art. Here are the fifteen from Twilight of the Republic.
Just the one Holocron on Geonosis.
There are seven of the cubes on the Coruscant planet.
Behind a grate on a building in the area to the left of the temple. Press a button behind to open it.
Behind a glass panel in the back left corner of the temple. To open it go to the top right corner of the temple and use the force to open a panel blocking a button.
At the top of the southern-most tall tower in the area to the left of the temple.
In the back of a shop in the top right corner of the area to the left of the temple, lower section. Shoot a target to open it.
Five of them on Tatooine.
Inside a building between the launchpad and Jabba's. Shoot a target above the door.
In a cave in the corner of the desert. same end as the sand village but other corner.
One in Naboo Space.
And one on Naboo itself.
Collecting all 15 of the Holocrons will unlock the Coruscant Library for the Toy Box and earn you the feat "Finder of Wisdom".