https://mathprize.net/posts/collatz-conjecture/ Not bad!

Only a few problems here but they are very hard, and they pay you $1 million for each one you solve: https://www.claymath.org/millennium-problems/

Only one person has solved one so far, and he turned down the $1 million. Go figure.

For programming, try rubyquiz.com. It’s supposed to be Ruby exercises and some of the problems are Ruby specific, but lots can be done in any language you want.