A water-themed alternative to the beach, and a good way to kill an entire day is to hit one of the water parks in the Phuket area. Closes relatively early–as it starts getting dark—so you can also plan to get back to your hotel for dinner and drinks later that evening. Here is a link to a pretty good article on one of the water parks.
https://www.timeout.com/bangkok/news/this-massive-thai-themed-water-park-in-phuket-is-finally-open-052422
