On Saturday, I was asked whether volunteering was a good idea. I was talking to high school and early college students about how to line up materials to get funded. Staying active and engaged in your area of interest is always a good idea, but just volunteering? Maybe not.
"Volunteering can look like a bad tattoo," I said, "something random that is slapped onto your resume with no context." If you are volunteering just to fill your cv with content, then it's probably a bad idea. But if the volunteering has something to do with a logical track of interests with which you can show continuity, then yes.
So take for example a student who has good grades but you see on their list of activities: volunteers at the homeless shelter, the wilderness society and did a 5k for the local teen center. You could possibly weave a narrative about how these interests overlap, but more likely, it just looks like someone who is volunteering just to check off that box.
Instead, think of something that makes sense contextually with your areas of interest and with the field you want to pursue in the future. It's always good to explain how your actions and passions intertwine, but having a coherent and logical train of extra-curriculars makes this plainly obvious. If you're volunteering for the sake of volunteering, take some time to think where you are headed with your career (or academic) plans and see if you could use your time more wisely.