Hello, it's exciting to hear that you're planning a trip to China! I'd be happy to supply you with some useful travel tips.

Best Time to Visit:
The best time to visit these cities in China is during the spring (April to June) and autumn (September to October) when the weather is typically mild. However, Shanghai and Guangzhou are also known for their warm summers, so this might be a preferable season for those who enjoy the heat.

Route and Accessibility:
All these cities have international airports and are well connected with each other by China's high-speed train network. For instance, you could start from Shanghai and take a bullet train to Beijing which takes around 5-6 hours. From there, another high-speed train can transport you to Guangzhou in about 8 hours, and finally to Shenzhen in just over an hour.

Accommodation and Booking:
There is a wide range of accommodations available in each city, from high-end hotels to budget hostels. It's highly recommended to book your rooms in advance, especially if you're traveling during peak seasons. All major booking websites should provide suitable options.

Clothing and Essentials:
Clothing largely depends on the season. Pack light clothes for the summers, warm clothes for winters, and layers for spring and autumn. It's also recommended to bring good walking shoes, as most attractions involve considerable walking. Don't forget your travel adapter that matches the Type I Chinese outlets.

Health and Safety:
It's advisable to purchase travel insurance that covers healthcare. Quality medical help is available in all these cities, but do remember to carry your own personal medication. Stick to bottled water for drinking, and avoid street food if you have a sensitive stomach.

Local Etiquette and Respect:
In all these cities, greet people with a simple nod or a hello ('ni hao'). Chinese people appreciate efforts to use their language. Avoid discussing politics openly. While visiting religious places, dress modestly and follow any signposted rules.