Building your own robots is a great learning experience. There are plenty of fun projects you can try to learn new things. This LEGO Domino Row Building Machine seems quite fun though. The developer used an old tape deck motor for the drive. A motor drives a pulley, driving a 8-tooth gear on the other end of a shaft. Every 5 turns of the pulley ends with a domino getting released. As the developer explains
The 40-tooth crank gear also drives a 24-tooth gear on the same shaft, which drives a 24-tooth crown gear. That 24-tooth crown gear drives an 8-tooth pinion which meshes with a 40-tooth gear on the main axle with the drive wheels on it. That makes for a 5:1 reduction from the crank to the main axle so that the machine releases five dominoes for every turn of the drive wheels.
The above video shows how this robot performs. More information is available here.