Location, location

I am looking to go to Thailand for 8-10 days in September. Can someone please give me some advice on the best place to base myself where I can find plenty of relaxing beach time but also a bit of partying and other travellers to habg out with?

Ko Pan ngan off of Ko Samui by ferry.
http://www.kohphangan.com/

yes! phangan and samui ialand is good place for information http://www.samui-center.com/

BASE: Phuket
RELAX ISLAND: Koh Yao Noi
PARTY ISLAND: Phi Phi
ALTERNATIVE: Krabi/Railay

On any given one, there's rock climbing and sea kayaking. Phi Phi has diving and snorkeling. Phangna Bay connects them all and has a lot to offer too. Also, Phuket's big enough to have something for everyone.

Yep, Railay/Ton Sai, Phi Phi and Phangan. Suggest maybe Thong Nai Pan Yao for a good combination of nice beach, relax and a bit of party on Phangan.
More info on these places in the link below.