I had written before on this, but Sveta (the wife) pointed out that I should mention the same Hermitage, Peter and Paul Fortress (it's really a citadel), Peterhoff, and possibly the Engineer's Museum. Be warned, the Hermitage can't be seen in one full day, even on a special Diplomatic Ambassador's tour (which she had me following around when I first came because of it's translation to English). There is also a small and less known museum exhibit of some of the strangest things in the world across from the Hermitage. It's always recommended by the locals in St. Petersburg because the exhibits are not to be seen in the regular museum.

Oh, and if you like shopping, definitely visit the Hotel Viru in Tallinn, Estonia. I have to walk to each of it's 70 stores for certain sales seasons. I only remember the January season, but I know there's another in the summer.

Enjoy your planning, it'll be a great trip. Sky in St. Petersburg, Russia