I agree with the Hawaiian-tropical theme. You can get all the tropical party goods half-off at the end of summer (maybe even sooner). Give everybody a plastic lei, or a flower garland. Get some of those plastic serving dishes shaped like palm trees or fish or whatever. You might serve fruit salad or fruit kabobs in a cut-open pineapple.
If it's your 13th birthday on the 13th--that makes it your "golden" birthday. You could make gold your theme if that's the case.
But you could still use the gold theme to emphasize you are changing from childhood to teen years; make it a big deal, ask guests to bring baby photos and try to guess who is who; have a trivia question game about the biggest events of the last 13 years; maybe play up the fact that 13 is a prime number and you're in the prime of your life. (OK maybe that's too silly.)
Always fun at a birthday party is a Polaroid camera or a digital, if you have one.
You are smart to think about this early. Good Luck and have fun! I remember my 13th birthday party -- vaguely -- it was xxx years ago. :)