Dear [имя],
Thanks for your letter. Sorry I haven't been in touch for so long, I was busy with my school. That's awesome that you're going to visit Russia!
Well, I think that you can do a lot of things in two days, even though it takes much longer time to explore the city. You should definitely see Palace Square and walk on Nevsky Prospect, Also you should visit Peterhof and Hermitage and, it's really interesting there.
Have you been in Russia before? Aren't you afraid of cold? It's pretty cold in winter in this city. Are you excited for your trip?
I have to go now. Hope to hear from you soon!
Best wishes,
Andrew