Phuket Attractions

You'll be able to book activities and day trips via the many travel agencies around town. Prices are around 1000+ baht.
You can join a package tour that takes you to Phang Nga province north of Phuket for a day of rafting, ATV, and elephant-riding/show. It was a full day of activities but felt a bit like a tourist trap. If watching animals perform depresses you, avoid this type of tours.
Spend a couple of hours horseback riding on the beach. A guide will lead you and have the horse go very slowly if it's your first time.
Try water skiing and wake boarding! Basically you're in an ugly artificial lake and must pull on a moving overhead cable to power you forward. It's a bit tricky at first but could be quite fun once you get the hang of it.
I thought the canopy tour, flying over treetops via zipline, was not very exciting. It was good fun, but the view was not overly impressive.
Spend an evening watching muay thai boxing.
FantaSea ( official site ) is a theme park where you can spend an evening seeing exotic animals and an extravagant dance show involving elephants