The best thing to do if your time is limited is to study in depth where you want to go and talk to a local guide operator who can possibly help you tailor your trip to see everything you want to see in the amount of time you have. Maybe start with a list of sights and then have the operator help you create an itinerary - this is what I did for my trip to Gansu with China Odyssey Tours.
I had four days (within which I needed to travel to Gansu Province and back to Shanghai) and the UNESCO-listed Mogao Grottoes and the Gansu Provincial Museum in Lanzhou. I told my tour coordinator that aside from these two places, I was open to seeing other things in the area and asked them to make suggestions. They came up with a very satisfying itinerary utilizing the short amount of time I had. With me on the trip, I had six friends. All were long-time residents of Shanghai, Chinese speakers and very interested in Buddhist art. They were the best possible group of travelers with which to go to Gansu.
See my four-day Northen Gansu itinerary.