The best ways to signal intelligence are to write, say, or do something impressive. The details depend on the target audience. If you're trying to impress employers, do something hard and worthwhile, or write something good and get it published. If you're a techie and trying to impress techies, writing neat software (or adding useful features to existing software) is a way to go.

if you are asking about signalling intelligence in social situations, I suggest reading interesting books and thinking about them. Often, people use "does this person read serious books and think about them" as a filter for smarts.