Shanghai's a great city, but not really all that different from Hong Kong.

You can get many, many places in China within a 3hr flight (at least those places with nonstop service, which if you can leave from CAN is a LOT of places).

Some suggestions:

Go to Guilin (KWL) and take a boat down the Li river to Yangshuo for a change of scenery. Yangshuo transformed from a fun backpacker town to a cheesy Disney-China 5-6 years ago (and Guilin gave up on having any identity other than as a tourist attraction a decade before that), but the boat ride remains one of the classic must-dos on any China itinerary. You can also rent a bicycle in Yangshuo and pedal through rice paddies surrounded by bizarre limestone peaks (or hire a pedicab or moto-rickshaw if you're not up for biking). It's absolutely beautiful.

You can get 1hr nonstop flights for under $200 roundtrip from SZX or CAN (several flights a day) or even ZUH (on some days). I think there's a nonstop from HKG on some days, too, but it's more expensive.

The Terracotta Soldiers in Xi'an (XIY) are on most must-see lists. The tourist guantlets that surround the excavations are horrid, but the soldiers themselves are very impressive, and Xi'an is an interesting city to wander.

Of course Beijing (PEK) offers tons of sightseeing opportunities. Between the big things (like the Forbidden City and the Great Wall) and the sheer fascination of the varied street scenes in different parts of the city (from the twisted hutongs to the massive Stalinist boulevards to the quaint campus of Peking University).

I'm partial to Nanjing (NKG), since I lived there for a year. It doesn't have the world famous tourist sites of some of the above suggestions, but it's a beautiful, pleasant city, quite unlike the industrial nightmares of Shenzhen and Zhuhai and whatnot. And there are definitely things to do.

Bottom line: go to your local bookstore and browse through the China books in the travel section. See what interests you. You're bound to find something.