Not to detract from the excitement, but it seems that after people find the chests for the first time, a great deal of the motivation and adventure of the game will be lost. After finding the chests once, I can only imagine people trying to find the fastest path to each one, making it inevitably easy for those who have played, and impossibly difficult for those who are new to the game. I'm sure it will be fun nevertheless!
EDIT: After thinking a bit more, only choosing 5 of the 20 might add a little spin to it. Who knows. Oh, I have a nice map for this but I am not placing the chests, and it's not all default bricks, sorry.