Flight
There are no direct flights from the US to Sanya, common connections go through Hong Kong since it is near Hainan Island, but you are of course free to book other options.
Travel Note
For US citizens travel to Hainan is Visa free for 30 days, travel to areas of mainland China (like Beijing or Shanghai) is Visa free for 10 days. You must have a return flight booked before you arrive. Travel to Hong Kong is Visa free for 90 days.
Hotel
This will be the location for wedding events, but you may stay at nearby hotels if you wish. Travel between hotels will be your responsibility.