In addition to many of the things already mentioned (driver's license, credit card, medical card in small ziplock; emergency info in iPhone that is accessible without passcode), I've shared my location with my wife in Google maps. Whenever she opens the app she sees a dot at my location, as long as my phone battery still has juice.

I also always carry an emergency whistle, ever since spraining my ankle while trail running in the woods years ago. I was able to limp out, but realized that if ever I needed to phone for help, rescuers might get close with GPS but still have a hard time finding my exact location.