What ship will you be on?
I did the Baltic cruise this past August with Princess. Booked my tour with Princess - the 2 day Grand Tour & loved it.
There are 2 very good tour companies that people rave about here - Denrus & Red October. Both have websites to get info.
If you scross down in this section you'll see some topics on St. Petersburg that will be very helpful. Listen to the info Stan has written - excellent.
Any specific questions I'll try to respond & help.