I went a long time ago, but found one of those tiny maglite torches really useful as back streets were badly lit and often quite potholed.
And if current advice is still not to drink the tap water, take that very seriously - no ice or teeth brushing with it - the local nasty is pretty nasty!
I loved it there. I loved the Russian museum as well as the Hermitage. Also Pushkin's house is really interesting, and there is a little covered market nearby where you can marvel at 1000 different types of sour cream, honey and cucumbers.
If you can get a trip to a banya in some way, do it, it's great. Perhaps try to organise through your hotel or a tourist group. My Russian friends didn't seem to be all that keen on the public ones, thought they might be a bit grubby.
it's also worth having a bash at learning Cyrillic. It's not actually that hard, and it will make you feel super cool.
Oh and if you are out late on any of the islands, make sure you check the bridge opening and closing timetables, you don't want to get stuck. And learn the difference between the different Baltika beers. One is a nice lager, one looks exactly the same but is actually rocket fuel.
I am very envious.