If you choose the car tour. It is 2-4 hour city tour that usually includes such sights as: the Square of Arts with the Michael's Palace, the Nevsky Prospect (the main street of the city), the Anichkov Palace and the Anichkov Bridge, the Fountain and the Moika rivers., 
the Kazan Cathedral, the Admiralty, the Winter Palace, the Palace Square, the General Headquarters Arch, the Senate Square with the Bronze Horseman (monument to Peter the Great), the Palace Bridge, the Point of the Bazil Island, the Mosque, the House of Peter the Great, the Aurora Cruiser(we can go inside), Peter and Paul's Fortress (If you wish we may visit it with entrance to St.Peter and Paul's Cathedral where the Russian emperors are buried), the Smolny Convent, the Taurida Palace, the Foundry Bridge and the Foundry Prospect with the "Big House" (the KGB headquarters), Church on the Spilt Blood, Michael's Castle, the Summer Garden, the Field of Mars, 
St. Isaac's Cathedral, the monument to Nicholas the First, the University Embankment with its historical buildings - Kunstkammer, the Academy of Sciences, St. Petersburg State University, the Palace of Peter the Second, the Menshikov Palace, the Academy of Fine Arts.
But in warm day it's better to take walking tour. It will include the most interesting places of interest, because almost all of them are close to each other. So usually it includes Nevsky prospect, Church on the Spilt Blood, Cathedral of uor Lady of Cathan, Michael's Castle, the Summer Garden, the Field of Mars, St. Isaac's Cathedral, the monument to Nicholas the First, the University Embankment, the Palace Square with Winter Palace (whish houses the Hermitage museum), The magnificent veiw of the Point of Bazil island, the widest part of Neva river and Peter and Paul's fortress. We can walk to Peter and Paul's fortress inside
.
I'm sure, if you visit this tour, you'll never forget the beauty of Saint-Petersburg.
It can be car or walking tour.
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