Comment by stonemetal

On just about any project basic bug fixes would be good. You know the easy noncritical stuff that tends to get overlooked. Fix up the slapdash UIs open source apps tend to have. Tests, beat on a library's interface in a way that someone too close to the code base might not think of. Depending on the app, data import\export might be a good place to start. It rather depends on one's area of interest.