Can I get visa on arrival if fly into HKG?

I am a US passport holder & plan to fly into HKG & would like to book a very short tour of Canton & Shenzhen area. I understand I do not need visa to fly into HKG, but how about my tour?? Can I get exampt from visa, or how can I get in to the tour from HKG?? YOur site said I can be exempt if book tour to this area, can I book this tour in HKG???