Peterhof and the Church of the Spilled Blood were my two favorites. Catherine's Palace was right up there, too. Going through immigration was a breeze, despite the reminders for patience from the ship personnel.
We spent an extra day in Stockholm so we could see the Vasa Museum- amazing! We enjoyed Old Town, also. The great thing about this being the last port is that we took the airport tour so got to see the city, and then were dropped off at the airport. Our hotel (Radisson Blu) was located in the airport and was even in the terminal we flew out of the next morning!