The beaches and pools were quite deserted during the day, as is to be expected in China. However there were a number of people sitting by the pool in the shade. There was more swimming at nighttime. For water activities, the Hilton has a water sports center that had different types of boats. Our friends go there to golf. The Yalong Bay Golf club is very nearby. And of course there's the spa. If you have children, the Hilton has "Kidz Paradise" room (babysitting) that looked like it could keep kids busy for awhile.

As I mentioned, we didn't do any sightseeing, but my here's some things on Hainan that look interesting (from Lonely Planet and a Hilton document).
- Tianya Haijiao: 24km north of Sanya. Literally 'edge of the sky, rim of the sea'. People like to get their photos by the rock that's pictured on the back of the 2RMB note.
- Ximao Zhou is an island visible off Sanya's coastline. Go for hiking or snorkling.
- Scuba diving is offered by a couple of companies in Sanya.
- Jianfengling Nature Reserve: 115km from Sanya. Many different species of plants and insects.
- Monkey Island (Nanwan Houdao)near Xincun: Ferry to the island and visit about 1,000 tame monkeys.
- Near Tongzha, visit Li and Miao minority villages.
- Nanshan Buddhist Cultural Park
- Nanshan Guanyin on the Sea: statue of the Goddess of Mercy that is taller than Statue of Liberty.
- "Small and Large Caves" aka Wonder of the Sea and Mountain.
- Qixianling Ridge and Hot Springs: hike through the forest to the summit then return and soak in the hot spring.
- Qiongzhong: 984-foot waterfall and a market.
- In Haikou: Tomb of Hai Rui, Five Officials memorial temple, Sino-Portuguese architecture around Xinhua Lu, market on Bao'ai Bei Lu.

For roundtrip airfare, we paid 2680RMB (cash) each via e-long.com. It was more expensive since we booked last minute and the lower fare wasn't available Shanghai to Sanya. The flight cost was the same as another travel agent had quoted. I'd suggest checking c-trip as well. Also, I've seen the Shanghai expat magazines advertizing really low-cost packages to Sanya that include air and hotel.