Well, the Terracotta Army's in Xi'an, which is a 12 hour train journey away from Beijing - or a 2 hour flight - so I'd probably stick that in the list of things to do generally in China rather than Beijing specifically!
Go to the Temple of Heaven. It's incredibly impressive in itself, but I especially loved how it's still full of Chinese people just playing cards or music along the walkways in there, and it's a great little haven away from the city.
Go wander around the hutongs and get yourself completely lost. I'd recommend the streets just to the south of Yonghegong Temple (which has its own tube stop, and is worth a visit in itself if you've nothing else on - peaceful temple on an 8 lane ring road sums up China for me), or Nanluoguxiang slightly to the west of that, which is an interesting street of boutiques and art shops, and seems to be a sight to see in itself for many Chinese tourists. The Bell and Drum towers are around there too, though I've only been past them. Hutongs are basically just old style Chinese courtyard housing, still in use today, with their own little shops and communities. They're not much to look at as buildings, but you can lose yourself in there for hours - awesome for buying chuanr too (barbecued meat on a stick for about 10p).
I guess the Olympic venues should be on the list too - I assume they're still being lit up at night, but I've not been there for a few months now. They're big.
For nightlife, I tended to go out in Chaoyang, but the most typical (I guess) places are Houhai and Sanlitun. Plenty of decent bars all over the place, and I'd recommend
CityWeekend to look up anything specific going on.
Regarding the Great Wall, think about which bit you head to - there's several different sections which people get recommended to visit. I went to Mutianyu, which was brilliant, and almost empty when we visited, as the holidays had just finished. Somewhere like Badaling has been nicely restored, but it's the most heavily touristed, as it's closest to the city, so it's harder to just step back and appreciate the scale. There's also a number of different walks you can do depending on how adventurous you feel, over parts which haven't been restored (around Simatai, I think - my friend had twisted her ankle, so this wasn't really an option for us).
Do you plan to go anywhere else in China, or just stick around Beijing? There's plenty to do in the city itself, and the sheer number of people everywhere can make things a bit tiring, but there's a lot more to the country too
