In the SF area - my parents and my bf and I went on a dinner cruise on the Hornblower yachts. That's a thought, pretty romantic around the Bay.
[Edit] It's not hard to find jewlery under $500, but I think that it might be more worth it to make a real day of it that she'll never forget. How about picking up cheap tickets and fly her to France or something just for lunch.

Definitely on the order of a unique gift.