I know you are going to have a great time in St. Petersburg! Looks like many people are considering tours there. Private, ship's, 6 people, 8 people, two day, one day, evening -- there are so many choices.
We had a great time in St. Petersburg, touring with DenRus in a private group of four close friends. Taking a private tour enabled us to move at our own pace and choose what we wanted to do. We particularly liked the option of "Lunch with a Russian Family." We also took the time to tour a local market and see the huge case full of caviar!
Of course we also did the Hermitage, Catherine's Palace, Yusopov Palace and the subway as well as Stolle the famous pie shop.
It might help you in planning your tour to check out our trip report and pictures at http://www.bully4.us/stpete.html
Have a GREAT cruise!