A couple of other things I tend to mention too:
If the radio starts talking in our ears please stop talking to me as ATC might want to tell us something. When you have more experienced passengers they can become very proficient at hearing the aircraft callsign and muting themselves effectively!
If you fly from a grass airfield then it is always worth explaining that the takeoff and landing might be a bit bumpy - especially if you have someone who hasn't experienced any form of aviation away from long strips of concrete.
Also know your aircraft. I did a lot of my early flying in a C177 and it had a tendency to be really quite nose-heavy at low speeds (eg: as you flared and settled on the runway). This could translate to the nosewheel coming down quite hard - even with full back stick - and a mention of things like this can stop someone who isn't aware of it being suddenly worried.
The earlier comments about stall warners, a/p disconnect etc. warnings are other excellent examples of this. I'll brief that the aircraft has lots of things that beep and complain in the headsets - they are all normal, and specifically mention that the stall warning will make a noise just as we touch down as this one does un-nerve passengers.
Paul.