The weather in Beijing and neighboring areas is notoriously fickle, so make sure you dress appropriately. In the summer it's hot; travel with sunglasses, sunscreen, and a wide-brimmed hat. It gets terribly cold in the winter, so dress in layers and pack gloves, a hat, and a scarf. And if you plan to do any hiking, make sure to bring sturdy, comfortable shoes.
Also, checking the weather forecast before an excursion is always a good idea for last-minute wardrobe changes. Don't carry too much cash or expensive jewelry. Other things to bring along? A camera, a change of clothes if you're staying overnight, and your common sense.