I highly recommend Math Olympiad (MOEMS)problems. All the elementary level problems can be solved using elementary school math, but kids have to work hard to solve them - even mathematically gifted kids. The problems provide a level of problem solving found in few if any math curricula (public school or homeschool).

http://www.artofproblemsolving.com/Books/AoPS_B_CP_MOEMS.php

Summer