Dear Monica,
(before opening) How's everything. (opening 2 purpose) I'm indebted to you for your company in the summer camp, which will be a treasurable memory in my life.
(body gain) During the one-week visit to the university in the US, I was impressive in such extraordinary experience. For instance, not only could I deepen my understanding of American education, but, immersed in (= exposed to) the English environment, I achieved great improvement in my listening and speaking skills. Moreover, to my most excitement, I made lifelong friends like you.
(closing 1 thanks) Thank you once again, (closing 2 expectation) and, if you may be interested in the Chinese culture, I'm thrilled to invite you to join the summer camp in China next year. Looking forward to your positive reply.
Yours,
Li Hua
