Many people leave their ID documentation at home, including their driving license, passport, etc.
I now carry a photograph of all of these important assets around with me in my iPhone. Normally I'd recommend @evernote as a good place to store this stuff, but as the app isn't password protected, I feel better keeping it in one of the many photo apps which requires a password before access is granted. The photographs are encrypted and, once transferred to the app and deleted from the standard iPhone photo library only accessible to you.
If it's something like a library, gym or swimming card, featuring a barcode, the receptionist may even be able to scan the card directly from your phone... carry your card with you the first few times just in case.