People setting off lanterns- they are great fun and fascinating for kids. You can get some great ones on ebay- very impressive and quite long duration. I think they must get up to about 1000' judging by duration and rate of climb. Once in the higher winds, they are away. Had a spectacular one that caught fire and descended rather rapidly.
I would like to think some twerp, somewhere, convinces himself that he's just seen a UFO!