As HKT Boy says in his post above...........depends on what you like to do.
Off the top of my head there is Elephant trekking, snake farm, Phuket Zoo, Fantasia, Go-Karting, shooting range, bungy jumping, various day or half day trips to other islands including snorkelling, canoeing at Phang Na, and the island tour that can be arranged by your hotel if you want is worthwhile. A day trip to Phi Phi island is usually considered a must for newbie visitors and can be had for 1600 baht upwards from the roadside kiosks. Be prepared to bargain for good deals on any trips. Just be happy with what you pay.
A lot of the above is also dependant on the weather as the island trips really need decent weather to be a good experience.
As for nightlife, Bangla Road is the main centre of attraction for tourists. Check out the lady boy soi opposite Shipwreck bar, wander up Soi Eric and meet Nicke from this forum at Mai Thai bar. He will introduce you to other board members who will probably suggest other activities not mentioned so far in this thread.
Enjoy your trip!