Math professor Harold Reiter has some essays on "Exploring Fundamental Concepts In Elementary Mathematics" at http://www.math.uncc.edu/~hbreiter/Zoom/ConceptsIndex.htm . "The essays are based on the idea that mathematics should can be developed well when students are motivated by rich exploratory problems that are easy to understand but require the development of mathematical structure to solve."

He teaches at Math Zoom http://www.mathzoom.org/summer-camps/summer-camp/faculty , a camp for gifted math students, and has written problems for MathCounts. He lists many math resources at his site http://www.math.uncc.edu/~hbreiter/ .

"To see what is in front of one's nose needs a constant struggle." - George Orwell