Tiny blue cubes are hidden throughout the Star Wars Play Sets, collecting them unlocks pieces of Concept Art and eventually a toy for the Toy Box. Here are the fifteen from Rise Against the Empire:
There are four Holocrons on Tatooine.
Star Wars Concept #10
On top of Jabba's Palace.
Star Wars Concept #11
Through a force door behind Jabba's Palace.
Star Wars Concept #15
On top of the giant crashed rocket.
Star Wars Concept #1
In a cave beneath Jabba's Palace, requires a Bantha to get in.
There are 5 Holocrons on Hoth - although one of them is during a space mission.
Star Wars Concept #5
Behind a tech panel on the upper layer of the inside of the base.
Star Wars Concept #3
In a cave beneath the Wampa cave in the base, requires ice smashing.
Star Wars Concept #6
Inside the Wampa cave beneath the radar tower out in the field.
Star Wars Concept #4
In an ice cave above the exit from the base to the field.
Star Wars Concept #7
In the "Escape From Hoth" mission, in front of one of the space slugs.
Forest Moon of Endor
Three on Endor.
Star Wars Concept #13
On high ledge with a gliding platform on the far side of the Ewok village.
Star Wars Concept #9
In a cave between the Ewok village and the Imperial base leaving through the right exit.
Star Wars Concept #12
Inside an AT-AT, use a Tech panel on the walkway.
The Death Star
One inside the Death Star and one outside it.
Star Wars Concept #2
At the end of the mission do not kill all of the Stormtroopers, instead jump up on the ledge around the edge and use a Super Jump to reach.
Star Wars Concept #8
During the "The Battle of Yavin" mission in the dodge section before Darth Vader's TIE Fighter evade section. It's behind an orange and blue structure, go over the top of it.
The Second Death Star
And one in the flying section as you leave the second Death Star.
Star Wars Concept #14
During the "Death Star Destroyer" mission, as you fly out from shooting the core this is above one of the pipes almost immediately.
Collecting all 15 cubes will unlock the "Imperial Probe Droid" toy for the Toy Box and the "Relic Hunter" Feat.