Every time I go camping, at the end of my stay at a given site I take my children through the campsite and we pick up every little bit of trash we find.
When we first started doing this, my kids would focus on 'our trash'. I explained to them that we would pick up all the trash, even that which isn't ours. It's one small way, to leave the campsite and the world a little better than we found it.
Sometimes it feels like we're fighting the tide, but I think the habit - a little better every day - finding ways to make the world around you a better place, is time well spent.