Can NSF Broader Impact be an unrelated volunteer activity? I’m a graduate student preparing to apply for an NSF fellowship and I volunteer a few hours a week working on what I think is a pretty important cause. It has nothing to do with my research or science education (although it is supported by data analytics, though I’m not directly involved in that part). 

Does it seem feasible to present this work as my Broader Impact? 

 A: From the NSF's Grant Proposal Guide (GPG):

"The statement on broader impacts should describe the potential of
  the proposed activity to benefit society and contribute to the achievement of specific, desired societal outcomes."

and

"Broader impacts may be accomplished through the research itself, through the activities that are directly related to specific research projects, or through activities that are supported by, but are complementary to the project.
  NSF values the advancement of scientific knowledge and activities that contribute to the achievement of societally relevant outcomes. Such outcomes include, but are not limited to: full participation of women, persons with disabilities, and underrepresented minorities in science, technology, engineering, and mathematics (STEM); improved STEM education and educator development at any level; increased public scientific literacy and public engagement with science and technology; improved well-being of individuals in society; development of a diverse, globally competitive STEM workforce; increased partnerships between academia, industry, and others; improved national security; increased economic competitiveness of the US; and enhanced infrastructure for research and education."

In addition to addressing this topic in the GPG, the NSF also has a website on this topic: https://www.nsf.gov/od/oia/special/broaderimpacts/
Without knowing more about your volunteer activities, it is hard to say whether it fits these requirements.  If you are donating your time to feed the homeless, then although this a worthwhile cause, you may find it difficult to integrate into your proposal.  If on the other hand, you are volunteering at local schools to help increase excitement about mathematics, then that probably could be worked into your proposal.
The bottom line is that broader impacts is part of your proposal, so whatever you propose it should be integrated in a sensible way that reflects what is in the GPG.
Relevant Past MO post:  Broader impacts and synergistic activities
