We made up a problem with technologies that we use at work targeted at frontend engineers. So we set up a project with a Symfony backend and made the candidates create Twig templates and SASS styling based on Bootstrap. Then we put that up on Github with a README. So the candidates could focus on the frontend stuff without caring for the backend. Still they had to figure out how to set up SASS compilation and modularization of Twig templates.

As I said it's not very hard but you can get bonus points if you come up with considerations for responsive design or stuff like that. Overall the aim is to create readable idiomatic code and not come up with something clever that no one but you can understand.

I guess that's a very specific task that's of no great use to others but the point is to have a specific problem with some blanks that can be filled out with a reasonable amount of work by the candidates. Plus the general workflow using git and such.