I go to Beijing 1-2 times per year for work. Done most of the "touristy" things over the many trips there. Here are my thoughts...
1) Climate is like Houston in summer (Hot / Humid) and Ames in the Winter (Cold / Snow). Spring and Fall are very nice.
2) Avoid Oct 1-7. It is a week long National Holiday. Tourist sites will be very crowded.
3) Air pollution - Can be really bad in Beijing. It is pretty rare to see blue skies and the sun without a gray haze. I have been there when you couldn't see more than 1/2 mile and could feel the grit in nostrils / throat from breathing. Masks are highly recommended on days like that. Rain and wind are the best things to clear the air of the pollution.
4) Traffic, traffic, traffic. There are cars, buses, trucks, motorbikes, motorized carts everywhere. Motorbikes and motorized carts don't tend to follow the rules of the road. Always look both ways before crossing as they run red lights and often go the wrong way on the street. Lane markings for cars are really just suggestions. You will see a lot of cars weaving between lanes.
5) Big brother is watching - no access to google or facebook on internet. Internet traffic is monitored and many things will be blocked / screened.
Tourist sites:
Great Wall - I recommend Mutianyu section. Less crowded than Badaling. Take the cable car enclosed Gondola to the top and walk down the wall. At the bottom you can either ride a ski lift down or a wheeled toboggan down an metal track. Can be spectacular if the skies are clear and you can see a long way down the wall.
Forbidden City - Very large complex with many buildings, courtyards, and statues. Architecture is very interesting to see. Many artifacts from all throughout the Chinese dynasties can be seen in the different buildings. Very easy to spend 4+ hrs wandering thru the entire complex. Note that you will exit Forbidden City on the far north end which is the opposite side from Tiananmen Square (long walk back...)
Temple of the Heaven - Very interesting architecture and site. Can spend a couple of hours looking at the couple of buildings and surrounding gardens. There is a good shopping area just on the east side of the Temple of Heaven.
Shopping: Be prepared to barter. There are no price tags on anything. Know the exchange rate and what you are willing to spend. Right now 100rmb = $15. They will start ridiculously high. Start low (20 - 25%) and don't be afraid to walk away. If they are making money on the sale they will come after you or reduce their price. If they don't come down it probably means that you are too low...
Olympic Venues - Bird's Nest Stadium and Water Cube Natatorium are very interesting to see in person. They are already starting to show their age and they are less than 10 yrs old...
There is a lot of history in / around Beijing. It is a city going through a pretty amazing transformation over the past 20-25 years since their economy has opened up. Beijing doesn't have the super tall buildings like Shanghai but they are starting to create their own skyline. You will see areas that are very westernized and then others that are older and look like eastern Europe / Soviet Union style buildings.
TLDR: Lots of great stufff to see, very crowded and polluted. Have fun and enjoy the sights.
