I don't think it's unreasonable to get a babysitter for volunteer time! I think it could be a great way to get an afternoon to be a woman in her own right-- especially if you are a SAHM. My baby is still too young (2mo, barely), but soon, I plan on finding a babysitter for 2-3 hrs at a time once a week or so to do things like go to the gym and just be myself for a spell. I'd do more time, but...$$$$, you know?
But if you really don't WANT to be apart, I think canvassing door to door would be ideal. It's not the same as direct charity, but here in Colorado we are trying to pass a tax increase for P-20 education, and so I plan to volunteer for them during the "final five" days of the election, at least.