Trying to get to the Great Wall and back in 8 hours is an invitation to miss your connection. Head for town. If you're not carrying bags with you, I wouldn't bother to hire a taxi for the duration. Get some Yuan from the ariport ATM (get and keep the receipt so you can change back what's left), take a reliable guidebook, ignore ALL the touts inside the terminal building, go outside to the taxi rank, tell him to take you to TianAnMen square -- and start from there.

The guidebook should have Chinese characters describing place names so you can point to where you want to go. I found Frommer's Beijing guidebook to be far more realistic than the tired old Lonely Planet.

Taxi between PEK and the center of town is about Y90 plus a Y10 toll each way, if you get into a taxi marked "Y1.60" (more comfortable, a/c works), a bit cheaper if you get into a taxi marked "Y1.20".

If you end up buying enough schlock to need a backpack, there are so many phony North Face backpacks for sale that the local expats call them "North Fake." I limited my kids to 4 "Rolexes." We also ended up with some Foakley sunglasses and all sorts of phony Mao-era memorabilia, plus 4 battery-powered thingies that are supposed to seal plastic bags -- or at least they did when the guy was demonstrating them. Plus a "Burberry" scarf for $1!