I thought it unusual that no-one offered you suggestions ... Ive just come back from 10 nts in Patong, and for me, it wasnt long enough to do all the things we wanted! Apart from all the usual touristy stuff (most of which Bobmac has suggested and is all good) you may find some other stuff to do on this great website www.cathyandgarystravelpages.com
A drive around the island is a must - just hire a driver, they are really cheap. We only did the south end of the island (read my recent jbr's) and went to Wat Chalong and all the viewpoints, absolutely beautiful scenery. We did Phi Phi Island day trip, but didnt manage to do the James Bond canooeing one, which we really wanted to. There's also a "Hong by Starlight evening cruise" which is meant to be unreal. If you like the cultural themepark scene, then Fantasea is a must - its amazing, not only for kids. If youre interested in the "ladyboy cabaret" show stuff, then Simon Cabaret is meant to be good, another thing we didnt get around to doing. We also wanted to go into Phuket town for a look around, but didnt do that. The nite weekend markets in there are meant to be unreal and very very cheap. The new shopping Centre in Patong called Jungceylon is worth a look at too, plus its airconditioned!!! Def the elephant ride is a "must do" we did our trek with Siam Safari and they were gr8, but we only did the 2 1/2 hr trek, not all the other stuff. The elephant trek at Kok Chang is meant to be good as well, recomended by many forumites, and its also on cathys website. Have a good look at her site, as she lists lots of things to do, places to eat, shop etc.
As Bobmac suggests, for a 10 nite stay, Id try and spend every 2nd day relaxing by the pool and chilling, otherwise its just all too tiring!!!
Kitty