Welcome to the forum Duncs.
Have you been to Thailand before, especially during Songkran?
If not, for a first timer, Songkran is great fun. There will be a couple of days where you cannot travel around without being soaked by both locals and other tourists. It is very hot and humid around this time so get a yourself water pistol from the street sellers and join in the fun. Remember to put your wallet, camera, phone etc in a plastic bag before going out.
For info on the girl rates have a look at this thread:
http://www.phuket-info.com/forums/nightlife/46475-girl-rates-patong.html
What to do and see? Well, there are numerous good beaches outside of Patong such as Surin, Laem Sing, Kamala, Karon Kata, Kata Noi and Nai Harn. All worth a visit as are others. There are ladyboy shows at Simon Cabaret, a Thai themed show at Phuket Fantasea.
You can go elephant trekking for around £5, snorkelling at Coral Island, day trips to Phi Phi island and James Bond Island.
There is a zoo where you can sit with a tiger for a photo, crocodile show, a snake farm, a plenty of temples to see.
That should start you off.