St Petersburg is a more young and friendly city, than Moscow. After a 7 hour train journey, we arrived in a city with English signs as well as Crylic. What a blessing after Moscow.
the Hermitage is an overdose of culture. Think La Louvre with oppulent decor. It could not be done in one day. See what you can take, then think about a second visit.
The Fortress, however is a two day afair, which you can use your entry ticket to go back to. Too much to write about, but go see!
The Spilled Blood cathederal is spectacular, similar to St Basils on the outside but amazing inside. Completely covered in murals. The little market to the side is worth a visit.
St Issacs for the views, if you can take the steps. It says 200, but does not mention the extra 50-60, outside. It is also amazing on the inside.
Stroll along Nevsky and admire the lovely girls, tourists and occasional war veteran beggar.