How about bug fixing? Give someone a repo with a tricky bug, ask them to figure it out with the help of their coding agent of choice.
It doesn't have to be a "tricky" bug. A straightforward bug will do. If it's too tricky, the logic could be better off being rewritten.
It doesn't have to be a "tricky" bug. A straightforward bug will do. If it's too tricky, the logic could be better off being rewritten.