My kids are 6, 5, 3, (almost) 2 and (almost) 2 months. Because I have the younger 3 which are quite limited in abilities as far as helping,and I don't want to be 'in the way' as far as keeping an eye on them, we do things that don't involve large crowds, or any crowds at all.
We pick up garbage from local parks or on the sidewalks as we go on walks, bake baked goods for bake sales (fund raising for local organizations, good causes,etc.), and consider how we can help individuals with things they may need help with (shoveling snow, picking up items from a store, helping people move, etc.) instead of getting involved in a place where I and the younger 3 may be too much in the way and slow down the pace of things where the older two could probably help out.
As my crew gets older, it is a goal of mine to do some sort of public community service on a regular basis where we can all truley be a big help. I want to develop a heart of service in my kids, but I do think that can be developed by helping individuals as opposed to always doing something public... Just incase you're concerned about the little one 'being in the way' as I am with my youngest 3!