US citizen flying into HKG for about 30 hours, and would like to stay in a Schenzhen. Need visa?

I'm a US citizen and passport holder. I'll be flying from Seoul into HKG for 30 hours or so in August, before moving on to Thailand. I'd like to stay in Schenzhen, but am unsure if I'll need one for my stay. If I do, what is the cost/length of the process?

Would I be able to go back and forth between Hong Kong and Schenzhen without problem?

Thanks!