When I was farming full-time, I used to find one every year or two while I was disking in the spring. 

Usually there was no note or address attached.  However, back in the '90's I found a balloon that did have a card that was enclosed in a ziplock freezer bag.   There was a little explaination on the name on a card and a Church address in Kansas.   It seems a Church youthgroup was sending out ballons for a little contest to see who's got the farthest.  I sent the card back with a note about how and where I found the balloon and a Scripture message.   The little girl got a prize at Church for third-farthest distance.   I received a nice letter of "Thanks" and a little hand-made beaded trinket that the young girl made herself.   I still have that trinket up on my office bookshelf.