The keyring seems to be the item most in demand judging by comments on this thread. However, Airbnb sent me one a few years ago (looked like the one below) and it didn't last long at all. In fact, it fell apart faster than any other keyring I've given guests.
The thing that alarmed me though was that this tiny, quite flimsy item was sent via courier (Fedex, DHL or similar, can't remember which) all the way from the US to the UK. If I had realised that, I would have requested they didn't sent it. Think of the carbon footprint!
More recently, I was sent an Airbnb throw. I haven't used it yet, as I couldn't figure out where it would work in my listing (colours are turquoise and peach with white trimmings), but it looked okay. This time, it seemed to be sent from an address in the UK, but I have no idea where it originated from.
If you're after these sort of items, I have seen similar keyrings on sale on Amazon (doubt they are authentic) and people selling the actual Airbnb throws on Ebay.