great info PTravel and everyone else.
So I will go directly to the hotel. Are the currency changers at the airport open that late (since I wont have any yuan to pay the cab). My initial plan is as follows:
Get in on the 16th midnight to a starwood hotel in Beijing. Will spend 3-5 days there and then go to Xian for 3 days/2 nights and come back to Beijing. Fly back to the US early morning on the 26th.

Beijing Itn: Mutianyu Great Wall, Forbidden city, Tienanmen Sq, Birds Nest/Water Cube and shopping (probably will stay at the Westin or St. Regis - free nights to burn).

Xian: Terracotta Warriors, the great mosque, city walls - plan to stay at the Shangri La if its affordable (heard the Sheraton there is pretty old)

Depending on how quickly I get done with Beijing, I may take trip to Shangahi on the express train and then fly from there to Xian.

Question on clothing: I was planning on jeans/shorts & Tshirts assuming its going to be pretty hot/tropical weather. Sandals or sneakers or both? Is there anywhere I will need dressier clothing (no business related items on my itn; just sightseeing)?

Question on schedule: based on the above itineraries, any suggestion on number of days I should spend at each location and is the trip to Shangahi worth it (if so, do it before Xian or after)?