Patong beach is good fun, there's nothing much to do in Phuket itself.

You can rent jet skis for about €50 for 3 hours.

You can also go wake-boarding all day for about €20.

Patong is absolutely mental at night! The things you see are unreal.

Go snorkelling on Phi phi and take the night trip to the different beaches and islands. Brace yourself for the dirt though.

I loved northern Thailand. Chiang Mai especially. It's really worth the trip, internal flights are very cheap!