Last year my son wore a Medic Alert bracelet that was sized so he couldn't get it off. It was from Medic Alert and had the phone number for the organization, so someone could call and get our contact numbers, etc. We worked a long time for him to hold his wrist up to "a mommy" if he was lost, then since he isn't speech delayed simply say "help me". I didn't want him to say he was lost and I thought if "the stranger" thought there was medically something wrong there would be more chance they would get him somewhere safe. Also, I thought less likely they would kidnap a child who is in a medical crisis (or so they may think). Any paramedic would know to call the number on his bracelet. This was all great until we had to get the bracelet resized and it is now just too big enough that he can get it off and he doesn't want to wear it. Before he never seemed to mind and other kids remarked that it was neat. That is my #1 recommendation if you can get it to work.
I have debated what to do for our August trip. I am terribly frightened of him getting separated. He knows no strangers, also. (I think with so many of our kids, esp., they have so many teachers and therapists that they think all adults are there to help them. JMHO) He is also a wanderer. Anyway...none of his shoes have laces, we use the slip ons. I think I am going to make adhesive labels on the computer (maybe the size of a shipping label), that say "I have autism. If I am lost please call XXX-XXX-XXXX" I won't put his name on it or any other information and stick it on his back. I had thought about the waterproof issue until reading this thread...maybe clear contact paper over the front, so the adhesive still works. I am also hoping this may cause people to think before making comment when he has to use the GAC.