Any ideas of what to do from after breakfast at end of tour to our departure time at 7:30 pm (air)?
Kinda depends on what you will have done and seen by breakfast at end of your tour.
Assuming your breakfast ends at nine, you should have about 6 hours to spare - that's enough to get to an attraction or two, provided that they are within the city limits.
It also sorta depends on your interests. Some of the things that I wouldn't mind seeing again and that don't seem to be included in your tour: piskarevskoe memorial cemetery (30-min drive from downtown); Faberge museum (smallish), Russian Museum (huuuge); yusupov palace (if you're into the whole Rasputin story); museum of the Navy; Artillery museum, the Aurora tour; museum of political history...
Where will you be staying? And what is included in your tour? I can then let you know what I especially enjoyed
If you are in the RS tour and staying in his tour hotel, they'll guard your luggage and let you use their lounge while you come and go to different things on that final day. Nice to know if it rains.
If you are into art museums, the Hermitage is both an art museum and a palace. The standard tour is the palace. In addition, there are miles of standard art galleries. Also, the great Impressionist collection is in a separate building.
The two hour canal tour by boat was way too short.
I agree with PerilsofP on the recommendation of piskarevskoe memorial cemetery (burial site for those who died in siege of Leningrad in WWII) and the Russian Museum. The Russian Museum is the largest collection of Russian Art in the world with many artists who you may have never seen before. I really liked the Russian Impressionist painters I saw there. I strongly recommend budgeting time to use the subway returning to the hotel. Traffic at times was horrendous so I wouldn't depend on a quick taxi ride back to the hotel. Good luck, Tom
If you appreciate art, I'd recommend you to visit The Hermitage Museum in St. Petersburg as it is not only one of the oldest museums in the world, it is also one of the best. It is located inside the Winter Palace of Empress Catherine. In fact, the Russian Tsars managed to build such a large collection of priceless artwork that it won’t even fit into the huge Winter Palace. There are many, many outbuildings and even an archive you can visit.
There can be long queues in front of the museum. I recommend you to book a guided skip-the-line tour.
We have spent 6 days in the Hermitage and it was well spent. We did about two hours with a tour from a river cruise and if that had been it, it would have been so disappointing. If I had a few hours in Petersburg, I'd spend that at the Hermitage with the Rembrandts, the room of generals and of course there is a whole building -- General Staff Building -- that contains 20th century art. Get tickets at the General Staff Building or else from machines in the courtyard of the main building to skip the line (which is made up of locals who need to show ID to get free or cheap admission and clueless tourists who think they need to stand in line)