Even though I'm an adult, I'm known to pull out the bucket of Legos and play fairly often. They're great for sparking the imagination--and that's now true in the kitchen, as well.
The Rolling Cookie Cutter from Lego allows you to make cookies in the shape of Lego bricks. The roller cuts cookies into 2x2 squares, one of the standard sizes for Lego pieces, with each cookie measuring about 1 inch by 1 inch. The Rolling Cookie Cutter uses plastic for the actual cookie cutter, mounted on a stainless steel handle with a silicone grip.
The Rolling Cookie Cutter can be a little harder to use than a standard cookie cutter: you have to get the dough to just the right thickness to be able to easily cut through it, while still keeping the dough from sticking to the roller.
While shortbread or sugar cookie doughs seem to do particularly well with the $9.99 Rolling Cookie Cutter, I think it would be fun to experiment with different colors of cookie dough--you may not be able to build anything with your Lego-shaped cookies, but some colors could make the cookies even more fun without having to decorate them.