I'd do Xi'an, Beijing or Guilin, in that order. Xi'an is probably at outer limits of your flight length, Beijing is over. Still, if you haven't been to any of those places, I think Xi'an and Beijing are definite must sees. Xi'an is more "doable" on a weekend, I think Beijing could use more time. The Li River cuise is the thing to do in Guilin, as others have mentioned, however, you have to check on when you are going as the water level drops in the summer months and that might hamper or even cancel the cruises; city is also interesting to walk around, not as frantic as other Chinese cities, at least on weekends.