If you've ever been to the Great Wall before, skip it this time. I found out the hard way that, once the initial awe wears off, the Great Wall is a major pain in the arse -- an all day deal, and then you've got to climb up eighty kazillion stairs to boot. If you can already say you've been, do something fun instead -- go to Wangfujian or something.
Key phrases to know:
Wo tingbudong -- I don't understand
Mei You -- we don't have it
Qing ni zai shou -- could you say that again, please?
Qing ni bangzhu wo! -- Please help me!
The first one is the most useful, I've found..
Kanyikan Yazi (Duck Hunt)
Beijing Yuyan Xueyuan, class of '87
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*quack*