Another good idea, if you like to explore some fine city, like Petersburg for instance, is to book flights to Russia. Since several years, we can find nice deal on planes to that country, also to it Asian area.
Moscow or Petersburg are localized at the North part of it, so winters are very chill in there. That’s why, the best season to explore this metropolis will be summer. From June till September weather is fantastic, days are dry and very warm, ideal for sightseeing. And two cities have plenty of nice monuments, so you have to decide which one you better like to explore, before you reserve your flights to Russia.