I would take a domestic flight to Shanghai for a long weekend. Beijing is awesome because of all the history and national importance (Forbidden City, Tiannamen Square, several Bhuddist Temples (the
Temple of Heaven was probably my favorite), the
Temple of Confucius, etc.), but Shanghai is truely a world-class city on par with New York and Chicago, IMO.
If you have the time and money, I've heard
mixed reviews about Yangtze River cruises.
If you're going inland near Xian to see the Terracotta Warriors anyway, I would go
here, too. Looks awesome.
These are areas I'd really like to visit if I ever am able to return to China.
If there is one piece of advice I'd give you, it would be to be an adventurous eater while you're there. As in any foreign country, steer clear of bottles of water with broken seals, but if the locals are eating at street carts, you should too. You'll be rewarded immensely if you stay open minded about the food, though you may break out in hives and your hands and face may swell up. And this may continue for months after you return. At least that's what happened to me. But who cares. Best. Food. Ever.