Guangzhou is a relatively safe destination, but foreign travelers are always susceptible to petty crimes. Pick-pockets and purse-snatchers are actually pretty common, so use good sense at all times, by locking valuables in the hotel safe, leaving jewelry at home, carrying limited amounts of cash, and storing that in a money belt. Also avoid walking around alone at night, especially in and around the train station. The good news is that serious crimes against foreigners are rare, partly due to the fact that the penalties are quite severe—including execution.
In terms of health risks, protect yourself from stomach upset by passing on the local water, ice, and produce, as well as any uncooked foods or unpasteurized dairy products. Stick to bottled water, and consider bringing an antidiarrheal medication with you from home just in case, since the quality of drugs in China is often questionable. Many travelers also return home with pneumonia or influenza, so consider getting an inoculation before you go.
Finally, ensure a pleasant visit by respecting the local traditions as much as possible, and refraining from voicing opinions contrary to China’s laws and customs.