MATHCOUNTS is a really fun competition. It covers various math topics. He will send newsletters monthly to all members.
Here are some topics:
Number Theory
Properties of Integers
Angles, Polygons, and Circles
Perimeter, Area, and Volume
Ratios and Proportions, Sequences and Series
Linear Equations and Functions
Quadratic Expressions and Equations
Coordinate Geometry
Counting
Probability and Statistics