I have a 5 hour layover at the Beijing airport and would like to do some sightseeing.

To be honest, 5 hours are too tight to have a good visit in the city. If you really wish to explore the city, I suggest you rent a taxi for a round trip to have a look at the Tiananmen Square and the surroundings.

it depends on timing, if you are arriving at day time then places are different than night time :)

I think 5 hours is too tight. If you really have no more time, you can rent a car and a native English-speaking driver, and show you the most famous sightseeing.

If you will, I can be your driver and your guide in. I've bought a BMW 320 one month ago. It's more comfortable than a taxi. And the keypoint is, most of the taxi driver in the city cannot speak English at all.

I am not a full-time driver or guide. In fact, I am a software engineer. But I like make friend with people all over the world. And, of couse, I can make some money. If you rent a BMW from a car rental shop you have to pay over US$200, not including oil, insurance and driver fee. However, you can pay me only US$200 for a BMW car, an native English-speaking drvier, and I will pay the oil and road toll myself.