What to do in St. Petersburg

As promised I’m continuing my series of blogs about un-missable tourist destinations on the different ports of call we send you all too. Bear in mind this is aimed at people who have never visited these ports before and it’s all based on my personal opinions and my favourite sites. If you feel I’ve missed something important out or disagree please make a comment below to help other cruisers make up their minds.
So on to business, today I'll be looking at St. Petersburg.