It is fun to see this thread come up again. It has been a few years since the last time someone made a thread with this subject.
There is this: you can show them things that help them understand that table tennis can take athleticism and skill. So many videos including ones already posted would be fine. One video I like for this purpose is this:
Another I like is:
But the ones already posted would be fine too.
However, despite the fact that this sport can be played with an extreme level of skill and athleticism, you can also be in pretty bad shape and still play TT at a very high level:
That is Richard Dewitt playing Adam Hugh. At the time Adam was about 2500. Richard is about 2300 in this video. Despite how badly out of shape Richard is, and despite how little he moves, he still gives a 2500 level player who is in excellent shape a lot of trouble.
So, many people who look at Rich play may think, "that is not a sport."
So, my thoughts on the subject are that TT is both a sport and a game. And it can be played as though it is either one.
Should you show your friends things that help them see the complexity of TT? That is up to you. But it is not hard to show some cool videos. Maybe you show and don't say anything about the reasons you are showing. Let them see or not see.