If you want something to commit a year of your life to, you want to match it to your own personality. It should be enjoyable (or at least satisfying) to you. It would help to know more about you. What are your interests and hobbies? Do you have any skills/skillsets that could be valuable to a charity?
Also think in terms of force multipliers. If you've experience with multi million dollar contracts, you could help a lot with a big charity, but would be wasted on a small one. Conversely, if your experience is smaller in scale, your help will have FAR more of an impact in a small, local charity. In a large charity, you would just be a cog in a different machine.
E.g. If you have some admin skills (and enjoyed it) you could be a life saver to small, local charity, that can't afford a full time admin person. If you have electrical wiring experience, you could put that to use.
Also pay attention to optics for your job. In a perfect world, you want something that looks good for them to be supporting, and builds skills you can bring back into your workplace. You don't want to be seen as swanning off for a year having fun, with little to nothing to show for it.