The Continental Mathematics League sells books containing an accumulation of their most challenging questions presented to CML participants in the previous years.
My dd's mentor suggested she works on these problems. My dd is really enjoying working on them. They are word problems and some of them are fairly challenging for kids their age.
A sample question for 4th grade is:
"The numbers 10 A 16 B C 25 are in order. The letters A, B and C represent different numbers. The difference between 10 and A, A and 16, 16 and B, B and C, and C and 25 are all the same. find the number C represents."
The sell the booklets for about $20 (provide the answers on the back of the book) I believe they have them from grade 2 and up.
We also loved the John Hopkins CTY math although we only have time for that during summer.
