If you want to have a vote that actually counts, consider jury duty.
Jurors have the right and duty to judge not only the facts of a case but also the validity of the law even though it is no longer common for a judge to advise jurors of this fact.
by Richard Sutton
In the early United States jurors were commonly understood to have the final say regarding laws by having the option of acquitting. This ability gave the citizens of the states a way to send a message (acquittal) back to Congress that the law in question is a bad law and that the citizens will not enforce it. Advising jurors of this fact became increasingly uncommon during the 1890's during the growth of the Progressive movement (the Prohibition folks, among other things...)
It remains true, however, that in the February term of 1794, the Supreme Court conducted a jury trial in the case of the State of Georgia vs. Brailsford. The instructions to the jury in the first jury trial before the Supreme Court of the United States illustrate the true power of the jury. When instructing the jury, Chief Justice John Jay said: "It is presumed that juries are the best judges of facts; it is, on the other hand, presumed that courts are the best judges of law. But still both objects are within your power of decision." "...you have a right to take it upon yourselves to judge of both, and to determine the law as well as the fact in controversy".
In 1895 the Supreme Court produced a decision that held to the same principle. In Sparf vs US5, the court ruled that although juries have the right to ignore a judge's instructions on the law, they don't have to be made aware of the right to do so.
As recently as 1972, the U.S. Court of Appeals for the District of Columbia said that the jury has an "unreviewable and irreversible power... to acquit in disregard of the instructions on the law given by the trial judge..."
I would add that if you decide that a law, as applied in a given case, is invalid it might be best for you to simply state that you are not convinced by the evidence beyond a reasonable doubt in order to avoid possible charges of contempt of court or jury tampering that have occasionally been applied to principled jurors for finding fault with the law (judges and prosecutors sometimes get annoyed).
The Fully Informed Jury Association has a wealth of information on this subject.
Supreme Court Literacy Test - Richard Sutton
McDonald v. Chicago and Second Amendment Incorporation - Austin Raynor
Citizens United: A Blow to Government Censorship - Austin Raynor