I've got the following on my USGI dog tag chain (think: "breaks if it gets snagged or pulled"):
-Small compass (really useful when travelling actually although I just bought for emergency use)
-Mini Fox 40 Whistle
-Photon LED squeeze light
-"dog tags" (why not? They were free, and do have blood type)

I'd like to add some kind of a fire starter but haven't figured out something sweat proof and perhaps some small edged cutting device/mini knife. I do carry a p-38, but it's on my key chain.