Xi'an really has very little to do and could be done in a day. The main attraction to come here for are the teracotta warriors which are about an hours bus journey out of the city.
Housed in three rooms the teracotta army is like walking into a real archelogical (spelt wrong) dig. Room one has the biggest collection of soldiers stood in formation seperated by mounds of earth. The next two rooms are smaller and look as though they are still being worked on.
The area offers little information about the history of the army or detail about their significance or design, so its worth reading up before you go. However, a film room showing a reinactment and short history movie is something to view once your there even just for a laugh.
In X'an itself it has fantasic market streets called the Muslim quarter, where you can buy anything from wierd street food to knock off watches.
Highlights of visiting is seeing the city walls and the Big Goose Pagoda which is a buddist temple.