Really liked St. Petersburg, but we were a little rushed there. Because it is part of Russia you need a visa to enter and we stopped there on a Baltic cruise. So, we HAD to arrange a guided tour with a company. Normally, I'm more of a let's explore on our own, but it was still really neat. We only had two days there, though, and that probably wasn't enough. Between the Hermitage, St. Peter and Paul's fortress, Yusupov palace (Rasputin), St. Isaac's cathedral, etc. it got to be, "Oh, look, another ornate room covered in gold..." after a while. Really cool city, but 2 days was not enough time to really see (and appreciate) everything.
Although, one of the most memorable parts was a guy with a bear cub on a leash in St. Issac's Square; don't see that everyday!