Sonds like a great trip. I went to China for a total of about six weeks spread over two years in the late 90's. I got to see the terracotta warriors of Xian. The museum in Xian is pretty good as well. The other cool thing is the wall around the city of Xian. It is about 50 to 70 feet high and is probably 4 or 5 miles on each side. Very impressive. My favorite chinese city is probably Chungking or Dailian. I wanted to visit the old port of Lushun(Port Arthur of Russo-Japanese war fame). It is on the other side of the penisula from Dalian. One neat thing in Dalian is a retreat that was originally built for Mao and other high party members. Absolutley gorgeous area. It is not buuilt on the vast scale of so many other projects in China. It is much more human and elegant. One other great place to go is in Sechuan provine near the city of Leshan. It is Mt. Emi. It is one of the four most holy sights to the buddahist. Absolutley incredible all the temples other structures that have been built on this 13,000 tall mountain.
You cannot know China without a visit to Xi'an, to see the warriors and the museum, as well as that city wall. You can see a little about it here: http://www.travelchinaguide.com/cityguides/xian.htm
In Beijing, Tien An Min Square and Mao's Tomb were the major sights to see, along with the adjacent forbidden city. After that, I liked Guilin best. The limestone hills that pepper the landscape make you feel like you are in the mountains, but the mountains are only a couple of hundred feet high - including in the center of town. It is a beautiful and fascinating place. Try this link: http://www.chinavista.com/travel/guilin/main.html