Isaac Cathedral
The Isaac Cathedral is very impressive.
The basilica is a great building, though you need to watch out for pick pockets, twice I have had a camera and a wallet stoleon a good tip is to take two wallets and more than one credit card, keep one in the safe in your hotel just in case of issues, fortunately I did just that or I would have had problems.
There are some first class hotels in St Petersburg including the Kempinski on Moika 22, which is my personal favourite.