Here's a guide to the imagination bricks: (there isn't one on the venture explorer)
AVANT GARDENS: the brick here is on the monument, after passing through the sentinel camp. travel up the YELLOW path until you get to the monument's head. once there, navigate your way around his neck to a platform jutting out of his shoulder. there should be another platform underneath the other one, which should be by a giant arm. use your right-click button to look-around and see that there is the imagination brick in the hand. jump carefully so you touch the brick, and you should collect it! if you fall and miss, repeat the process until you succeed.

NIMBUS STATION: head to the red-blocks arena where you meet Johnny Thunder from your faction missions, and see that there is the brick on the highest platform in the arena. you may need some friends to help you here, as you need to build all the same arena decorations (like spotlights, lasers or speakers) at the front of the concert. if you successfully manage to build all the same types, jump up to where the DJ is, and in a few seconds steps will appear and lock into position for you to climb. do so, and you should be on the platform you saw earlier, with the imagination brick on it. if you fall down, beware, as the steps only stay there for about a minute, so if they move away, repeat the instructions again.