Since Paul is most fond of cars, we went with automobiles. A true artist could probably create realistic trucks and cars on potatoes, but mine are basically recognizable. The white blobs are actually the potatoes, photography is the next skill I am going to try to pick up.
Brigham didn't get to stamp more than a couple, he was most interested in licking the paint off the stamp, and that really isn't part of the process!
After we put the body of the vehicles on, I went back and added the wheels. I think they are now distinguishable as vehicles, at least.