Husband & wife traveling to Beijing, Dalian! And Jiamusi july 20. What should we wear?

It is the summer time in China, so short sleeved clothes are enough. But you still need to prepare some long sleeved clothes when you go to Jiamusi and in case of the rain.
Walking onthe Great Wall needs about 3 hours, but if you are tired, you can choose to take the cable car. As for the Forbidden City, the recommended time is about 3-4 hours.

Imagine the weather in NC or LA. It's very similar. Dalian and Beijing are very cosmopolitain in dress but do try to dress modestly. Exposed shoulders on females are not common and being westerners will draw looks. A light blouse is always handy.
You can easily spend 3 hours on the Great Wall or in the Forbidden City. Remember to take bottled water and rest as required, it's not a race. Enjoy the scenery and the Chinese people, they are very welcoming.