Unless you get an individual visa which takes months to get on your own, you will not be allowed off the ship to wander in St. Petersburg unless you are with a tour. It is not like other ports or countries.

The Denrus tour we took with 4 other couples was one of the best experiences. For 3 days our split was about $300 per person. Our guide was so adept as getting us in and seeing places - she knew exactly how to get us in and what to see. If a bus was coming, she'd detour rooms and entrances and we actually saw so much. We actually even had a private tour at Yusopov's Palace - we were early and it was "lunch hour", she called ahead because we'd finished where we were and had an extra half hour. It was an incredible experience.