I did a one day tour with Emma Fan (www.emmastours.com) so not with the Westin. I was alone but you can arrange tours with larger groups. I had a flight out of Xian in the evening so I biked around the wall first followed by the Terra-cotta warriors.

Before being dropped off at the airport I visited the Tomb of Emperor Jingdi which is close to the airport. Personally I was more impressed with this museum than the main army even though they are smaller (1/3 lifesize or so).

Depending on how long you are there you can arrange for longer tours or customize it to see the things you want.

Be sure to walk around the Wild Goose Pagoda especially at night but as well as during the day. The fountains are a decent show and the locals like to congregate there to dance (maybe just on Friday nights and Saturday mornings since that is when I was there but hopefully they are out other times as well).