DS's favorite math has always been the Kangaroo questions; they're very visual-spatial and tend to require thinking sideways about a problem. They only require grade-level knowledge, but are quite different than anything he'd ever found at school. You can find old contest papers here:
https://kangaroo.math.ca/index.php?kn_mod=samplesOf course, if you wants LOTS of math, AoPS is the place to go - but there's likely to be a lot of learning involved, at any level you take
Not as easy to dip in and out of, but awesome math taught by people who find joy in it.
https://artofproblemsolving.com/
