Be interested in lots of things that other people might not find interesting. I think it's the way that I personally signal intelligence the most. For example, if someone has a herpolhoder on their desk, I try to ask intelligent questions about it. Or if the rain on the window is dripping in nice straight lines because of the screen occasionally pressing against the glass, notice that.
5sixes_and_sevens
Do something prohibitively difficult that not a lot of people are competent enough to do.
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.)