Great idea for lego storage! Instead of flat surface, add lego mats. Creative Solutions for Small Space Play: Underbed Play Table with Drawers by Great Little Trading Co. (does not ship to U. but think of the DIY possibilities)
How To Build A Fire Truck Bunk Bed theownerbuilderne. Kids love bunk beds, but you don't have to spend a lot of money to make them happy. You can make your own fire truck bunk bed, and make it a weekend bonding activity/project with the family.