I've been hosting for 2.5 months now with over 50 bookings. Get a keypad for your front door for about $100 (deadbolt) and just leave the room keys in the door for your guest to pick up when they arrive. I havne't had any guests steal my keys, but if they did, I would report it to Airbnb, change the front entrance keypad code, and make a copy of the room key.
That's a great question. You can also look at Keycafe. They have many locations in different countries. In short, you got to a Keycafe location, add a tracking device to your keys, and place your keys into the SmartBox. Your guests will get a message with the location and code. That way they can then get your keys when it's convenient for them. Here's more info about how it works **