I think my trip there could give you some ideas. We spent the 1st day in visiting the terra cotta warriors and horses and the Huaqing Hot Spring. That's really wonderful. We leave early in the morning by taxi and return to Xi'an almost at 16:00. After a dinner in the hotel, we went to watching the Tang Dynasty Show. It was really a rush day, but all of them were breath taking.
The 2nd day we were traveling in the city area after having breakfast. Around 9:00am, we were in the Bell Tower and then walked to the city wall for a short cycling there. In the afternoon, we were taken to the Shaanxi History Museum and then the nearby Big Wild Goose Pagoda.
In fact, I think that our 2-day trip is not enough for learn Xi'an. This city has so many places to go if you are interested in Chinese history.