Most likely, there is an established culture of [mathematical circles][1] in your country or even city; you should get in touch with your local "branch" for ideas about the scope and curriculum for your course. You might want to include your country/city in your question to get more localized advice.

There are many online resources, too, like [Art of Problem Solving][2].


Problem solving and olympiad problems usually works very well, while teaching a university level course on, say, complex analysis early usually doesn't. The mathematics should be elementary and easily accessible (good: elementary number theory, graphs, polynomials in one variable, combinatorics, Euclidean geometry; bad: differential calculus, advanced algebra) and the focus should be on the students *doing* mathematics instead of listening to it.


  [1]: http://en.wikipedia.org/wiki/Math_circle
  [2]: http://www.artofproblemsolving.com/