Here's the completed postcard with verbs in the present continuous:
Dear Sandy, Greetings from St Petersburg! We are here for the lovely Golden Autumn. Together with the residents, we 1) are enjoying the last days of sunshine. This is a quiet time of year, and so Betty and I 2) are enjoying our stay. Today we 3) are not visiting historical monuments. We 4) are relaxing in the gardens of Catherine's Palace outside the city. It is Saturday, so many families 5) are walking in the woods and 6) are having picnics on the grass. Betty 7) is resting. She 8) is drinking a cool drink by one of the fountains. I 9) am writing a few postcards. When 10) are you planning to visit this wonderful city? I hope all is well back home. Love Ann