One possible version：

I’m writing to you from my home in China. I returned home safe and sound. When thinking of the wonderful days I spent in New Orleans, I just can’t help thinking of you. It was so kind of you to have provided everything to make my stay there such a happy experience. I’ve learned so much, not only English but also the culture. And I really enjoyed a great deal, the fun and laughter we shared with each other. Thank you very much. I’ll always remember this trip as well as your kindness.

Welcome to China when it’s convenient for you. I’ll be very glad to show you around. And you’ll surely have a pleasant visit here.

I’ll keep in touch and write to you later. Please take care!

题目分析：本文是一篇书信题材的作文，要求写一封信感谢李华在去美国期间Brown夫妇对他的照顾，同时要表达自己内心的感受，提出希望他们来中国的邀请，并且表示要保持联系。写作时候首先要注意篇章结构的合理安排。书信一般分为开始部分，正文部分和结尾。在开头提出写信的目的，正文要表述所列的要点，最后提出自己的希望等。此外，还要使用恰当的连接词使段落之间和句子之间有机地联系起来，保持内容的完整、连贯。可以用到的句型有:Thanks very much for …..; I can’t thank you enough for ….;I hope ….等。在时态方面要注意写在美国期间的事情时应该用一般过去时，其余部分可以用一般现在时和一般将来时。

【亮点说明】本篇书面表达语言简练，要点全面。作者在开头写自己安全地回到中国，并且表达了对Brown一家的感激之情，然后讲到自己在国外学习的收获，最后提出邀请，希望他们一家来中国旅游并且表示会以后经常保持联系。这种结构安排使文章层次分明，显示了作者清晰的思路。在文中作者使用了较多的简单句，用到了很多短语safe and sound、as well as 、show sb around 、 keep in touch 和take care等，这种表达使文章语言简单明了、通俗易懂。此外作者还用了固定句式：it is convenient for sb …; it is kind of sb to do sth ;not only ….but also ….等，有效地增加了文章的亮点。