Public parks often require much more attention than they actually receive. Volunteer opportunities at the park can be as simple as picking up trash and pulling weeds when you visit, or you can organize a larger-scale cleanup and rejuvenation effort with the rest of the community. Just make sure to consult the local parks and recreation council -- or whatever entity oversees your particular facility -- to gain permission before launching an event. This is a great choice because your kids will be able to see how their efforts directly benefit the community.
Check your local county office to find out how to get involved with this program, which allows groups to select a stretch of roadway to keep clean via regular litter removal. This one's best for families with older kids, as participants usually have to at least be 13 years old.