The right kind of sightseeing :) We have enjoyed the views of Kazan Cathedral, Church on Spilled Blood, cruiser Aurora, Peter and Paul Fortress and many other attractions. It's much more interesting than seeing it all from ground.
Crossing the Neva in heavy wind is an unforgettable experience too :)
The guides are very friendly and professional.
Duration of the trip is perfect. Don't be afraid that 6 hours is too long, time will pass quickly. And you will still have almost full day to continue enjoying the city.