WHERE TO VISIT
Nevsky Prospekt is the main street you will be visiting during your stay. There are many historical buildings, (The Hermitage, The Peter and Paul Fortress, Church on Spilled Blood etc.) cafes and restaurant nearby.
Peterhof is the place you should definitely visit. During our stay, fountains were not functioning and despite that the place was worth seeing. Starting from May till September, I think the fountains are active and it is more beautiful. Peterhof is placed just at the Gulf of Finland coast and that is another thing which makes it worh seeing.

WHAT TO DO
There are boat tours you can attend to see the city from the canals. However those trips were all in Russian, we could not understand a word. If you want to attend one, I offer you to choose the shorter one because the longer it takes the more boring it gets :)
As you may know, there are many bridges in the city. They open at different times at night and stay open a couple of hours. So you should see the opening and closing times of the bridges from you guide and plan your arrival to you other or the place you would like to go, accordingly.The most famous bridge is the one on the left. Both of its parts open whereas other bridges' only one part open. You should go and watch it while opening. It was opening around 1.30 a.m. and people were celebrating this by drinking champagne.
WHAT TO EAT
There is a book store on Nevsky Prospekt. The food and drink is good at the cafeteria of the book store. I really liked the place.There are also many sushi places in the city. We had a favourite but I don't know the name because of the different alphabet. I just remember the place was covered with red colours.
Have fun!