Please take into consideration the weather forecast, our climate is very different from yours, please be prepared so that you don't catch cold. St Petersburg is famous for quick weather changes, we may see 3 or 4 seasons in one day (sunshine, rain, wind might be expected).
In autumn and winter I always dress for a temperature about 10'C lower that forecasted. Humidity and wind-chill is a ... let's say a problem.
Think layers! In a way that you can take something off if hot, or put it on if it gets cold and windy.
Even in summer light scarf or shawl may save the day.
Bringing an umbrella is almost always a good idea.
Comfortable shoes is a must!
In summer don't forget the sunscreen for your face, the sun here is very active.