For a Cessna 172:
"Before we begin, a few safety things. Please keep your seatbelts fastened at all times, there's really nowhere to get up and go during the flight anyway. Your seatbelt works about the same as any car seatbelt you've been using since you were three years old; put the buckles together like so, release them like this.
It's a non-smoking flight; if you must smoke, please step outside. In the event of a fire, the extinguisher is here. Release it like this, pull this pin, point at the base of the fire and squeeze.
Exits are here and here. Pull the silver handle and push out. Don't open doors until the airplane comes to a stop, don't open them in flight, and if you do leave the airplane, go out past the wingtips and don't come back for anything, no matter what you see or hear.
In the event of a water landing, take your shoes off and get clear of the airplane. Don't take anything with you.
If you're not feeling well during the flight, speak up. I'll change altitude or speed, open vents and windows, and do anything else I can to help you feel better. Breathe deeply and look as far away from the airplane, toward the horizon, as possible. Don't close your eyes. Sick sacks are in the seat backs and tucked over head. Open and use like this.
If you see anything that you think needs my attention please sing out. If I don't respond right away, point. You'll have my undivided attention. It may be another airplane, it may be this airplane; whatever you think is important, I think is important. It never hurts to have extra sets of eyes looking.
The airplane is noisy. You're welcome to wear earplugs. Remember to yawn or chew gum as we go up and down.
Please don't touch the flight controls or throttles, here, and here, during the flight. I prefer not to have to break your fingers.
Please don't disturb me during takeoff or landing; these are busy times, and it's best that I focus on the flying.
If you see me reading and muttering to myself during flight, this is normal. It's a bit complicated, so I do have to read the directions.
Our flight attendant will be around shortly after takeoff with cocktails and headphones for the inflight movie. We apologize for the selection this flight; it will be 'Batman Forever.' Please hang on, and have a good flight."
The flight attendant bit is optional, of course. Especially as one doesn't normally have on in the 172...but humor, even schmaltzy humor, goes a long way.
Passengers shouldn't just be briefed on how to open the door, but when and under what circumstances. Just as importantly, they need to know where to go after the airplane comes to a rest.
Reminding the passengers not to open the doors in flight or manipulate the door controls might seem like overkill, but it's not. I have seen curious passengers in light airplanes attempt to, or actually open the door in flight. I once dealt with a passenger who opened the rear doors on a Cessna 207 in flight; the aft of the two slammed open in the slipstream, crushing the empennage. I asked her why possessed her to pull the little red handle (which she actually had to bend out to operate), and she said "I wanted to see what would happen."
I asked her "did you?"
The pilot of that flight, a scenic tour, didn't specifically tell the passenger not to open the door in flight. He didn't see the need. He was wrong.
I think one of the most important things you can tell the passenger on the flight, is what to do if they don't feel well. Another is what to do if they see something amiss. Make sure you're clear.
Use humor where appropriate. It makes passengers feel better. Take all questions passengers may have, seriously.
Remember that while you may be in charge, and you're responsible for the safety of flight, everybody on the flight has the same stake in a safe outcome. Treat their stake seriously. If a passenger has a concern or doesn't feel right about something use the most-conservative-opinion-wins approach. If someone on the flight doesn't feel right about something, don't do it.
You are responsible for the protection of your passengers, and your effort at doing so includes briefing them in order to givem them the instructions necessary to be safe, and to enjoy the flight. Don't limit your self with only what's legally required. Cover the things that the passengers should know in order to have the best flight experience. It might include reminding them to turn off their flash on their camera, so it doesn't reflect off the window. It may reminding them to pop their ears on the way up or down.
The abbreviated briefing is more simple:
"Sit down, strap in, shut up, and hang on."