Phuket has a lot of activities to choose from, there is many Phuket travel guides around. You can go snorkeling, diving, island hoping, sky diving, water skiing, swimming and almost anything else you can think of. But remember to try and bargain the prices down as much as you can, the savings you can make with good bargaining skills can be huge.
Beaches
The major beaches from north to south are:
Mai Khao (Sai Kaeo) - near the airport, very quiet (aside from the planes!) and far away from it all
Nai Thon and Nai Yang - two quiet beaches in Sirinat National Park
Bang Thao (Laguna Beach) - long, very quiet beach
Surin and Pansea Beach - an up-and-coming upmarket destination
Laem Sing - small bay with stunning views, between Kamala Beach and Surin Beach
Kamala Beach - a quieter beach to the north of Patong
Kalim Beach - a series of small beaches just to the north of Patong
Patong Beach - the largest beach resort, known for its nightlife
Karon and Karon Noi Beaches - the second most-developed beach after Patong
Kata Yai Beach - busy, clean tourist beach with good surf
Kata Noi Beach - quieter sister of Kata Yai
Nai Han and Ao Sen - a quiet beach (probably the best) in the south, near Phromthep Cape view point
Rawai, Mittraphap and Laem Ka - set off point for lots of local islands, popular with locals for eating on the beach
Cape Panwa - home to Phuket Aquarium