For those who enjoy logic, math, and problem solving...
Link -
https://talent.math.wisc.edu/schedule/Each school year, the Talent Search creates five sets of five problems each and distributes them to high school and middle school students... throughout the world. These problems are unusual, challenging, and we hope, enjoyable. They are not easy, but their solutions do not require advanced mathematical knowledge–just talent in problem solving.
How to Participate -
https://talent.math.wisc.edu/how-to-participate/