Though perhaps I missed it in the foregoing, I have not seen here a briefing on the ELT, it is sometimes worth a briefing under some circumstances.
I brief all pasengers about operation of doors and seatbelts. As for controls, I brief passengers that I want their heels on the floor, and their feet ON the rudder pedals (if the seat is set so they could reach). I tell them that they will feel the pedals moving, and they are not to resist this movement. My reasons for this are two: I once had a passenger, whom I told to put his feet flat on the floor, get is rather large boot jammed, and it limited the travel of the pedal during landing. You will find in many planes that the location of an adult's knees, with their feet flat on the floor, is much higher than when on the pedals. I would rather over power feet, than have the control wheel jammed on their knees at the very worst time. As long as their heels are on the floor, their toes are not on the brakes.
Otherwise, the additional items I brief will depend upon the nature of the flight.
In addition to the foregoing, and particularly when flying over water, brief for unusual or inverted exit from the aircraft. In many aircraft, in particular most 100 series Cessnas, it is surprisingly easy to kick out the top of the winshield, and pop the whole thing out - great exit! This might be best reserved for later in the flight, or when a potential emergency shows the possible need in the very near future.
In addition, if passengers are new to you, what are they briefing you? Are you paying attention? Your biggest concern for non-emergency type passenger comfort are terrified passenger and airsick passenger. No passenger wants to admit that they are either. So what will they do? Nothing. They will sit still and say nothing. When you talk to them, they will not reply, or with very few words. If you have a very quiet passenger, they are briefing you, prepare to change something about the flight very soon, and probably you'll have to figure it out!
You're best passenger is a chatty youngster, they are doing fine, and you know they will pay attention to you if you demand it. (unless they are your teenagers - then forget it!)