It's an informal letter, and the tone is fine.Is this register and format appropriate or should I use more formal expression?
You recently returned home after staying in Britain for two weeks with the Morris family. You have now received a letter from Mrs Morris.
Dear Mrs. Morris,
I am writing to tell how I appreciated staying with you.You are such a wonderful family. The time which I spent with you all was great for me, indeed!
Firstly, next month I am coming back just for a couple of days. My friend Mike invited me to celebrate his birthday. Do you remember Mike? So I could pick up my forgotten stuff then. Is this OK for you?
Secondly, if I were you I would come here in May. The weather is pleasant and the most interesting places are not so crowded by tourists as at the peak season. Iíd recommend staying/that you stay in Prague. As I know, you are interested in architecture. Prague is an exciting city and offers many possibilities for sightseeing, such as the Castle, the Charles Bridge, Wenceslaw Square or the Old Town Square. If you like you can visit museums, concerts, theatres or clubs.
Lastly, it would be really pleasant/a great pleasure for me to meet you in Prague. I would like to introduce your family to my parents. They are really curious about you.
Thatís all for now. Once more, thanks for your letter. Iím looking forward to hearing from you again.