You can't do independent trips in St Petersburg unless you have an individual Russian visa. You are forced into doing the organised trips which are covered under a group visa. Don't miss the trip to the Hermitage. Anywhere else you can go independant.
You need to get your own visa for Russia. Red October, Denrus, Alla or Anastasiaworld are generally considered to be the best tour companies to go with in St P.
We have been told by AllaAlla tours and Red Oktober Tours that if we are arriving by cruise ship,staying for less that 72 hours and doing one of their tours we DO NOT need a vis.