Yunnan is a good place, I suggest you had better come in April and may
Xishuangbanna is the best tourist resort, where you can understand, and the culture of this part of the dai culture has distinguishing feature very much
You can experience the unique local life, taste of food,
As for the traffic mode, you can sit long distance bus from kunming
May you have a good holiday親愛的Mary，我很高興妳能來雲南度假。
雲南是個好地方，我建議妳最好在四、五月份來
西雙版納是個最好的旅遊勝地，在那裏妳可以了解傣族文化，這裏的文化很有特色
妳還可以體驗當地的生活，品嘗獨特的美食，
至於交通方式，妳可以從昆明坐長途大巴
願妳有個好假期
嗯，書信的格式妳就自己想吧