Well, if you have kids, you can always say that you are doing a project for school. That usually works. I also carry around a clipboard and sometimes a camera. With kids, you can also pretend you are playing hide and go seek if they are young enough. Another way is to say that you are on a treasure hunt.
One time we were geocaching at night and a polic officer came over adn talked to us and my dad started babling on abuot the gps and the police officer started going away in his car and my dad followed him adn kept talking....that sure got him away as soon as he knew we weren't doing anything wrong and we were ok!