1) VOTE! - If you are a U.S. citizen who is registered, you have an obligation to vote. People have fought and died for 236 years to get us that right in the first place, to expand it beyond white property owning men, and to it keep from being taken away from those who already had it. Make your voice heard! Even if you feel your vote won't make a difference to the presidential election, depending on where you live there are state and local races, initiatives, and constitutional amendments on the ballot.
2) If you can't vote because you aren't registered, today would be a good day to fix that so you'll be able to vote next time around. This site has information on how to register. If you live here but aren't a citizen, spend a few minutes thinking about whether you want to become one.
3) Consider voting for candidates from parties other than the Democrats or the Republicans, especially in county and city elections. I think adding more voices to the political conversation is a good idea, and at the local level they can actually make a difference.