We did dog tags.
WalMart has this machine that makes dog tags. Both for real dogs,

but also military-style dog tags. Oldest DS has been in a military phase for a couple of years now, so they seemed like a good choice.
Oldest DS's has his name, my cell number, and his birthdate. Well, there were three lines and I couldn't think of anything else to put.
Youngest DS's has his name, the word Autism, and my cell number.
I kept our GAC in my fanny pack in the little front pocket. I forgot to buy a lanyard before we left. I don't know that DS would have put up with it anyway. About Day 3 he didn't want to wear the dog tag (claiming the chain was itchy) and so I attached it to his fanny pack. He did understand that my phone number was on there and that it was there for if he got lost.
DS did have some trouble with the finger scans, and several times the CM just over-rode it, they were quick to pick up on it. I think (and I know this sounds silly) but if you've got a kid (even a big kid) and the parents are standing there talking very strangely (and no offense meant but I know my vocabulary is different when I talk to DS in a situation like that, DH and I standing there talking to an 8-year-old going "okay, honey, can you please put your finger right on this spot? no, stand right where you are and let Daddy put your finger on the right spot. And stand very very still for me please", this is just not how most people talk to 8-year-olds) anyway, most CM's are going to pick up on that.