Hmm, where to start? I hope it doesn't matter, for the sake of this thread, that I don't have an ASD diagnosis. Here are some social skills I wish I could've learned or been way better at as a kid:
I didn't learn until adulthood how to consciously bring about or hold onto a genuine smile. I learned to find the humor in little things, to find lots of positive little things to bring to mind when I need to smile on cue. Also, as a blind person, I've practiced smiling and touching my face during different kinds of smiles to see how they felt.
It would've been nice to know when and how to join a group, either for conversation or for games. I know this is awkward for most people, and I feel like the earlier we can teach kids about the social cues to watch for, the better.
Not seeming like a smart-ass, good one, I was often guilty of that one without knowing why. So, the art of being humble about what you know, gauging other people's interest, stuff like that.
Hmm, diplomacy/persuasion, I think that would start with why those skills are important, in case the kid is at a point ,in their lives when they don't care, perhaps some stories or having the kids talk about what they can gain or do for others by using these skills. Maybe skills like "I" statements, not directly giving others "should" advice, learning when and how to ask questions or state opinions, and stopping the automatic snap judgements in their tracks before they come tumbling out of your mouth.
How about empathy stuff, like not assuming you know how others feel, taking different perspectives to imagine different ways someone might feel in a given situation, asking questions about how people feel?
Finally, how about the give and take of conversation? Kids will be at way different points on this one: taking turns, gauging someone's genuine interest versus politeness, asking good questions, sharing appropriately, not going on and on for too long, learning to open up about yourself, ETC.
I know this is really long. I hope a few of these will be useful.