Looking for some resources for fun math problems. DS9 is already in school so this would be more for enrichment. There are only a few I know myself - I don't want to accelerate with this just add some depth and fun.

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=samples

Of 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/

My son loves Art if Problem Solving.
When he was younger (kindergarten), he enjoyed Bedtime Math. You can sign up for a daily problem via email. Sadly, the problems aren’t very challenging beyond the early level but they are always accompanied by a fun story/facts.