Probably one of the most underrated programming task is writing test. It's a pity though as it is one of the most satisfying programming task.
Tests are short, much shorter than the code they are testing. So you need a lot less investment to finish one.
Tests are (or should be) much simpler than the code they are testing.
Tests are the proof that the code still works the same way as it worked yesterday.
Tests can be super interesting, especially when mocking is involved.
If you are interested in contributing to an Open Source project, start by finding a small project that does not have tests or that has a low test coverage and contribute there.
If you are interested in the steps of contributing, try this explanation:
or this might be even better:
If you'd like to get help, comment here with the main programming languages you are interested in and if possible even with the project you'd like to contribute to.