I had to do a code test for a job (sigh). But it actually was kind of fun. The website takes your code and checks correctness and efficiencies.

The company posts "challenges" every month. But they also have lessons which contain some interesting code challenges. The only problems is 1) they're timed 2) sometimes the descriptions are kinda mathy which may put people off.

https://codility.com/programmers/lessons/