One suggestion is to visit the Forbidden City (map) and Tienaman square. 

Another option is the Silk Market, it closes around 8pm Beware, the vendors are very agressivie, but you can get good buys if you are prepared to walk away. If they ask 100 RMB, start at 20- you should be able to get 40-60% discounts on everything. Don't buy 2008 Olympic stuff there- it is all fake- only shops are in places (oddly enough) like the Forbidden City or the SUmmer Palace!

For a full day, visit Mutianyu.  One other nice thing about this section (vs Badaling) is that you can take a gondola (or ski lift) up and then come down gondola or, for variety, a huge slide! Don't waste your energy climbing up- you will have a hard enough time when you are up there as the wall goes up and down and the air is a little thinner than I am used to. It can take about 2 hours to stroll from the ski lift end to the gondola end.

 Night Market- if you can, go there and walk around a little. This is basically a line of food stalls offering mostly various different raw seafood on a stick- everything from live snails to starfish to octopus tentacles to scorpions (honestly!). You pick what you want and then they fry it up for you. People walking around with fried octopus tentacles on a stick- yummy! The market is a short walk from the north entrance to the forbidden city (I think, we were let their by local friends, so check with hotel). It is worth the visit if you can stand the smell.

Another option is Pearl Market, it is possible to walk away with beautiful pearls at a good price.