Advice wanted for VISA

(US Passport holder) At the end of May I will be in Hong Kong for a program through my college, I would like to extend my trip beyond the alotted time and transit into mainland China, maybe taking a train up to Guangzhou and spending a few days up there.

I am wonderring what visa I would need for this proccess. Is there an easier option or a visa on arrival for transitting across land borders?

Thank you.