In general terms, if you are just starting to think about contributing to Rust projects it is probably more important to find a project with a number of maintainers who are helpful with mentoring than to find something that is best simple and easy to understand. I have submitted pulls against projects with a single maintainer and had them ignored. In contrast, when i saw an issue marked as good for new contributors on the table for Bat, I had not one but two very helpful maintainers who guided me through it.
Once you do take the plunge, don't be discouraged if you are asked to make additional changes on your first pull request. This is actually a great opportunity to learn, and a sign that the maintainers are willing to take some time to help you.
So, to boil it down, look for an established project with a number of contributors and a well organized and active bug tracker. Lots of activity by a range of people means you are now likely to find some help getting started.