Hi Angie,
Wow, its not long until you get off!
Halkidiki will be warm but not hot in October...about 18-20 degrees. So it will be lovely but comfortable....alot like a warm autumn day in England.
Dress code is casual, although you may want to take something abit dressy incase you decide to go to a nice restaurant. Clothing wise, I would take light t-shirts, linen trousers, sandles etc. But pack a few cardigans as the nights will be cooler.
I hope this has helped. If you would like any further information, please let me know.
Warm wishes,
Cheryl, your editor