Volunteering in a campaign, running for office, designing and/or distributing pamphlets. All of these are potentially more effective than voting because you could get more than one vote for your preferred candidate that way.
And that’s just the obvious conventional stuff. If we get into theoretical strategies shit gets wild.
Minor nitpick: while participating in voting is an important component of politics, I don’t think it’s accurate to say it’s the most influential thing you can do. It’s not even the most influential thing you can do if you’re strictly operating in the framework of electoralism.