You may want to check out Rboy apps, they have an app called Rental Lock Automater (RLA) which I've been using for a while and am thoroughly impressed with it as it automates the entire process. Once a renter makes a reservation on airbnb, RLA picks it up, generates a lock code based on the last 4 digits of the phone numbers and sends a texts to it to the renter (I can set how manys in advanced I want the the message sent out). It automatically enables the code when it's time for them to check in and then disables it once the check out. Infact with the latest update (I have smart enabled my property), it even automatially turns on the A/C before the renter arrives and turns it off after they check out so it saves me a lot of money there.
You can check out more details about RLA on the rboy apps website, www.rboyapps.com
All you need is a SmartThings hub, a Z-Wave lock (like a Schlage, Yale or Kwikset - personally I prefer Schlage and Yale) and RLA.
If you're into home automation, I've written up more details about what you can do improve safety and reduce costs on another topic here:
https://community.withairbnb.com/t5/Hosting/Home-Automation-for-Airbnb-hosts-Home-Assist-custom-solu...