I understand that one could possibly save money by looking around without a tour or a guide, but with only 12 hours, time is not on your side. A guide could maximize your sightseeing and provide cultural insights that you wouldn't get by yourself.
I have a specific recommendation for you:
http://boards.cruisecritic.com/showp...0&postcount=12 I am not one to engage in unnecessary hyperbole, but this was one of the best "tours" I have taken, anywhere.