Ask HN: What are some good problems to code test a candidate?

Basically what the title says. The places I worked for all asked for a code test before inviting me over for a face to face. There has been some variety but the Toy Robot simulator (https://codereview.stackexchange.com/questions/96181/robot-toy-simulator) was by far the most used by the industry here in Melbourne.

I'm curious about what other people are using? I've heard of the Elevator simulator (https://github.com/donnemartin/elevator-simulator) and Conways game of life.

What else people are using that is easy to review, a good problem to solve and ticks most of the boxes you use for reviewing candidates?