I was in St Petersburg about 16 years ago. So, my recommendations are not so up-dated. But still, I'm sure that the transport network is as good as it was then. They have a very well developed metro net, all the palaces in suburbs are connected with commuter (RER- like) trains. It means that everything can be seen independently. To avoid the lines in Hermitage (you need at least one day to see it well) you can by the ticket by internet, I suppose.

My another recommendation, don't visit St Petersburg in winter. It can be very!!! cold and you will not be able to appreciate the sumptuous gardens of many palaces, like the one in Peterhof with its many fontains. The streets can also be very dangerous - ice can breake you feet, because the level of municipal services is still not very high.

In addition to many sights pointed here, I'd add Pavlovsk Palace and the palace in Lomonosov. Also, you MUST visit Petropavlovsk fortress and Mariinsk theatre. Don't miss Alexandro-Nevskaya Lavra - very famous historic cemetery which is as important for Russian people as Per Lachaise to French! To learn more about great Russian art not well known to the foreigners you MUST go to Russkiy Musey (Russian Museum) - with the help of the paintings you'll learn a lot about Russian way of life before the October Revolution, about Russian nature and so on. Don't miss the paintings by Levitan. Russian Naval Museum can be interesting as well.

Enjoy your trip,

Sincerely, Alex Elbert Jerusalem Israel