You won't be able to rent a car in Beijing because neither U.S. nor international licenses are recognized in China. This restriction should be cause for relief, as city traffic is terrible. If you want to get around by car, put yourself in the experienced hands of a local driver and sit back and relax.
The quickest way to hire a car and driver is to flag down a taxi and hire it for the day. After some negotiating, expect to pay between around Y600 and Y700. Most hotels can make arrangements for you, though they often charge you double that rate—you can probably guess who gets the difference. Most drivers do not speak English, so it's a good idea to have your destination and hotel name written down in Chinese.
Another alternative is American car-rental agency Avis, which includes mandatory chauffeurs as part of all rental packages—although this can also be very expensive, with chauffeurs alone costing Y235 per hour.
Avis. 400/882–1119; www.avis.cn.