What age group are you targeting? That changes where you want to start and what the final objectives are.
From you website it appears that you are targeting either high school or younger.
There are a lot of existing projects / competitions like First or Robofest that are designed to present the students with challenges that they can overcome and teach them via robotics - look at those first. Make it too easy and they don't learn anything. Make it too hard and they give up and fail.