I figured someone would have said this by now, and it seems obvious to me, but I'm going to keep in mind the general principle that what seems obvious to me may not be obvious to others.
You said efficient ways to signal intelligence. Any signaling worth salt is going to have costs, and the magnitude of these costs may matter less than their direction. So one way to signal intelligence is to act awkwardly, make obscure references, etc.; in other words, look nerdy. You optimize for seeming smart at the cost of signaling poor social skills.
Some less costly ones that vary intensely by region, situation, personality of those around you, and lots and lots of things, with intended signal in parentheses:
It was not intentional that all of these related to conversation. Maybe that's not a coincidence and I've been unconsciously optimizing for seeming smart my entire life.
It, of course, depends who you're signalling to. These sound to me like ways of signalling that you are intelligent to the unintelligent. (If that. They're good possibilities but I'm skeptical of about half of them.)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
(I plan to make these threads from now on. Downvote if you disapprove. If I miss one, feel free to do it yourself.)