针对六级考试的要求而言，除了个别用词及大小写的错误，你的作文总体写得还不错。我仅从英语信函写作的角度作一点个人点评，供你参考。

    这是一封写给一位外国教授的信函，因此写信时要注意婉转和礼貌，不同文化背景的人有时会落入陷阱，你的信也存在这方面的问题。具体讲解如下：

1. I felt thrilled to receive your letter, 这里要用feel, 不用过去式，虽然对你来说是过去发生的，但要让对方感觉你的兴奋现在依然存在，或记忆犹新，所以这里习惯上都用一般现在时。

2.  you can obtain the charming knowledge of Chinese history. 语气过硬，对方是个教授，你怎么知道人家需要获得这方面的知识呢？可以改为，you can be impressed by the splendor of Chinese history. 

3. if you decided to visit the great wall, you should fully agree with the old saying, decide要用现在时，should fully agree, 口气不婉转，本来should 的口气就够硬了，你还加上fully agree, 这成了命令了，岂有此理？此句可改为： Second, if you visit the Great Wall, you will be understanding the inner meaning behind a famous old saying in China — he who does not reach the Great Wall is not a true man.

4. taking your biology major into consideration, 表达有误，对方不是学生，不用biology major的说法。可以改为as you are a biologist 即可。

5. 你给对方提建议，通常不必说期待回复。可以说希望我的建议对您有所帮助。如果您需要进一步帮助，请马上联系我的意思即可。

附上我修改后的作文供参考, 请注意为了客套和婉转，我在有的地方添加了一些词，有些地方做了改写：

Welcome to Beijing! I am thrilled to receive your letter, in which you have mentioned that you are planning a sightseeing tour in Beijing. Regarding your travel, may I offer some suggestions, which I hope will be of some use to you.

First of all, I'd like to recommend the Forbidden City as your first stop, where I guess you may be impressed by the splendor of Chinese history. Second, if you visit the Great Wall, you will be understanding the inner meaning behind a famous old saying in China — he who does not reach the Great Wall is not a true man. Finally, since you are a biologist, I think the National Zoo can be a real find where you can appreciate the various kinds of Chinese animals there.

I hope these suggestions can be helpful. If you need further information about this, please feel free to email me.