Here’s an idea that I think is pretty cool, a robot that lays bricks. Not Lego bricks, but regular construction bricks.
There have been a few artists and companies that have actually commercialized this and create wonderful works of art. Here is another link to a different company.
Make Magazine recently did an article on some of the mathematics that is required to make some really impressive textures on some walls.
I have tinkered with some of the math, and it is pretty detailed. I hope to get into this project in the future (get in line, buddy). If I do get into this project I would probably use something like dominos or home made wooden bricks to mimic the bricks and I think a gantry style robot will be much easier to construct and program.