First, go to a travel doctor and get caught up on all shots!!
Shenzhen is an industrial city near Hong Kong- its where to (middle class) live and work. Its like Cincinnati but with a lot of rice patties and marshes. DO NOT GO TO THERE AMUSMENT PARK!!! There are some cool beaches there and sights from ancient China- you can get laid for about $20 USD!
Hong Kong is a fun place - its a global city like New York, Paris, London so expect to see a variety of people and cultures. Great food, but don’t eat where they tell tourist to eat, ask to eat at the best local places. They have a lot of knock off stores like Burberry, Louis Vuitton, Rolex, Cartier, Hermes, Tiffany etc. The are some stores that look totally legit down to the décor and employee name tags- but they are 100 % fake!! So be careful where you shop.
Shanghai - is a huge banking city on the water, lots of wealthy people from around the world live here-its the upper class/ upper society and much like Hong Kong, it is very global and diverse in culture and people- lots of Americans live here. Great food, shopping, stuff to do. Cool go-go night clubs and karaoke bars where hot girls dance in lingerie while some Chinese man is singing Fleetwood Mac. - You should defiantly check out the Swan Massage Parlor there- its not what you think but you do get to pick 2 scantily clad girls out of like 50 to give you a table shower, massage you, manicure, pedicure, facial and cleanse and you get to lounge around all day and use there steam rooms, saunas, hot tubs and exercise room- there’s no private rooms, its like one huge facility and everyone is wearing a robe. If you have time- go to some of the old Wushu and Wing Chun- kung fu temples that have been around since the 1500’s- they still practice there and there is one I believe where Bruce Lee, Jet Li, IP Man and a few others studied at-I do believe they are still open to tourist.
Beijing is a modern city, a lot of new infrastructure mixed with old. Its like ancient china with modern buildings with neon lights built around it. The Great Wall is there and there are lots of festivals and cultural rituals you can watch. A lot of people live here- traffic is a nightmare. 90% of people were a dental mask to keep from inhaling toxic air.
If you have time, go to Tianjin- its an industrial city with one of the largest ports in the world. Got lots of stuff to do. Got a few beaches and tourist attractions as well as where most of the Chinese Navy docks there boats. Its also home to one of the largest explosions ever recorded with a 4.0 magnitude on the Richter Scale. Cool bars and clubs
A couple other people have stated about street food- they are 98% right- Its ok in Hong Kong and Tokyo but China doesn’t exactly have an FDA that restricts certain chemicals and food that can cause certain illness or death. Eat what you know!
Most important- learn how to say no!- its spelled Bu and pronounce Bul with a silent l. The Chinese people see Americans as a bunch of Bill Gates walking around and want to sell you anything and everything! Just say NO!!