Saint Petersburg Travel Guide

If you're going to the Hermitage, be prepared to do some walking. I did not think a museum could be that big. Or that Catherine the Great owned that much stuff.

Fascinating city, crossed by a net of canals, like Venice ! Astonishing a night cruise on the river Neva.

It's one of the most wonderful cities I have ever seen!
I would not even know where to start with all the things that are there to visit, but first of all I'd suggest to go in the summer. The winter must be quite magical up there, but to see it well surely good wheather is better. Especially around the end of June- beginning of July, when you can see the "White nights" (when the sun does not go down and it's pretty clear even at 2 am).
Once you're there, and you have a guide (I used lonelyplanet, it's quite good as far as places to visit/opening times/tips etc are concerned, but don't trust the prices. When I went there, for example, it said one underground ticket was 8rubles, but in fact it was 12), you can move quite easily with the underground. Still, some places are far from any station, so also get a map of the city (you can find it in any bookshop).
I would also suggest that you should learn some at least basic word in Russian, because most people don't even speak english there. For example: to buy an underground ticket (it's acually a coin), ask for "adnà proyèzdka". Thank you: "spasìba", please: "pazhàlsta" etc...
Better not to use tram or marshrutki if you're not sure where they're going ;) but if you do, the ticket for the tram is sold on board and for the marshrutka you pay directly to the driver (the price is written outside the car).
Said this, my favourite thing EVER was the church (which USED to be a church, now it's actually a museum) of the Saviour on the Spilt blood - it may seem a typical old russian thing, in comparison to all the other palaces in the city, which are more european, but actually it was made as an imitation of the actually old ones, in the 19th century. It's built on the place where the czar Alexander Ist (or was it II, anyway) was wounded to death.
And finally, you  HAVE to taste khatchaupuri, a typical georgian thing. When you're walking along the Nevskij Prospekt, and you see a palace decorated with green statues on the first floor, take that perpendicular street and then turn right. After a while you'll have a "mama roma" (which is written like "MAMA POMA"), an italian restaurant (also good, and im saying it as an italian) on the right, and on the left a little georgian restaurant on the corner. Go there and ask for a  "hatchapuri s sirom" (khatchapuri with cheese). It's S-P-E-C-T-A-C-U-L-A-R!
Enjoy St Peterburg :)