The best interview we've used was sharing a simple but very not idiomatic Python file with working (but slow) code and tests at the bottom. The task was to refactor and speed it up. This allows seeing the actual thought process and some basic skills, while at the same time being something any good dev could do in 20min without much pressure.
I tend toward this, especially with some canned performance report. Sometimes I take something relevant from their GitHub and introduce a glaring bug, like an impossible dependency or infinite loop.
