Here are 2-3 possible demo scenes. All of them A+ in both picture and sound, with beautifully recorded music tracks. Paycheck's movie score was recorded by Shawn Murphy, arguably the best music recording engineer in Hollywood, and possibly the best symphony orchestra recording engineer working anywhere. He has worked extensively with John Williams (virtually all his of his scores since the mid 1990s), James Newton Howard, and (prior to around 1998) James Horner.

Paycheck
Chapter 12:
Start at about 50 minute point if you want to keep the length down. The selection starts with a conversation between two characters and ends as a subway train comes to a stop. It includes plenty of ambient sounds in a crowded train station, a shoot-em-up gunfight but no blood (the movie was directed by John Woo, and while it was one of his box office failures, he still knows how to choreograph an action scene), a can and mouse pursuit, a couple of standoffs standoff between two characters with the drop on each other (another Woo favorite) and a tense scene with the hero being chased by a train. There are one or two uses of the S-word here, if that's a consideration.

9
Not the recent musical, but the computer animated feature.
Two suggestions here:

From about 2-3 minutes into the movie (there's an impressive base undertone somewhere in that span) to the end of the battle with the beast that resembles a cat's skeleton. This sequence sets the tone of the story, includes plenty of ambience and deep bass.

Chapters 10 and 11:
Taken together these are nearly ideal in length and in forming a closed story. It begins with the characters moving through a wrecked and abandoned church where they are hiding out and ends after a pitched battle with a mechanized flying monster.


9 is definitely not animation for kids, and likely bombed in the theater because it's very dark (dark in its theme, not in its video). Probably the best post-apocalyptic sock-puppet film ever made. Lots of action but not bloody (how bloody can a sock-puppet get?) This movie is a full 16:9 image, with no black bars if you are using a 16:9 screen.


PS
I can think of two other conditions you might add:

1. The movie should be available on Blu-ray

2. The scene should not be the end of the movie, or give away the end, or include a major spoiler (such as a key character dies, or the ship sinks!). Unless perhaps the movie is relatively old--say, for example, 10 years. By then, if the viewer hasn't haven't seen it, it's unlikely he or she will be interested enough to ever do so, in which case telling how it ends won't give anything away. But in that case you should at least point out before the demo that there are spoilers. The age limit obviously lets The Natural slip into consideration (one of my favorites as well, by the way).