I recently started volunteering as a tutor for Reading Partners, a wonderful program for helping kids who are behind in their expected reading level. It's great because as a tutor you get to work with the same student every week and there's a structured curriculum to follow. In Seattle, there's a waiting list of kids who are in need of tutors, so if you're looking for a way to give back and have time during school hours, I'd definitely encourage you to consider becoming a tutor, too. You can read more about the program here.
(Also: the Seattle branch of Reading Partners has a wish list of books they'd like to add to their library!)