Hi,
While there is a huge Safety Sense leaflet on the CAA web site covering this, it does mention you have to use what is appropriate. So good question.
I work as a flying instructor. On trial flying lessons - and indeed all lessons - one has to be careful not to overload the student.
A trial lesson especially, since it is all new to them.
You need to be in control.
You need to let them know this, so they stay quiet when you ask them too.
They need to know what to touch, and what not to touch.
They need to know how to get in and out the aircraft, and how to adjust their harness.
They need to know where to go - ie safe arc, away from prop - in case of an emergency evacuation.
They need to know they can tell you what they think - ie if they are suffering discomfort.
(if going over water, you may want to add life jacket, life raft etc)
Keep it short, or they'll never remember anything useful - like how to open the door.
You don't need to mention engine failures, cockpit fires, airsickness, fire extinguishers, first aid boxes etc. I have done a first aid course. The box is next to useless if you haven't had training, and if you have had training, it is next to useless since you can improvise! (different for very remote areas)
Don't mention air sick! Tell someone not to think of a bright green elephant, and the first thing they think of is.....? So use words like discomfort!
Many will be producing large amounts of adrenaline at this moment, no need to increase the flow with stuff about EFATO etc. If you have one and crash, you have told them how to get out. If you have one and don't crash, then why brief. Even on a real EFATO, you can have a surprising amount of time!
I usually start with: Just like on a passenger aircraft, by law we are required to give you a safety brief. .....
On a weightshift microlight I then go: The emergency exits are here, here and .... wave all around. But you must be careful of the propellor - when it is spinning you cannot see it and it will chop you up!
I try and keep it serious, but lighthearted. Just like the flythomson video one with the kiddies doing the brief!