As someone generally not known as a “light traveller” I really did try when packing for Phuket to only take what was needed and was very proud of myself as I jetted off below my allotted luggage allowance. That said, I returned with rather a large amount of clothes unworn! Travelling to Phuket in December meant peak season in terms of people (a very popular destination for Europeans trying to escape their winter) yet one of the milder months in terms of temperature (although in reality that means under 30 degrees rather than over so hot as opposed to very hot). It is warm in Phuket all year round with temperatures ranging from 25 to 34 degrees Celsius. December sees the end of the monsoon season so one can expect these “milder temperatures” with lower humidity, ideally a cool breeze and calm seas.
But back to the packing... with such temperatures and a beach lifestyle one really only needs shorts and t-shirts, sun dresses, flip flops, swimming costumes and a comfortable pair of shoes for exploring and that is about all. You will spend your time on the beach (you can rent a sun bed from 100 baht) and if you can, bring your flippers and mask for snorkeling (although these can also can be hired). You will undoubtedly visit top tourist attraction Big Buddha and there are rather a large amount of steps to climb, which one is aware of in the heat, so always dress light. That said, be respectful of local customs so ladies will need to cover their shoulders when visiting local temples.
Do not forget insect repellant (this is key), but if you do there are little supermarkets everywhere so you can easily get supplies if you need them. Phuket is very relaxed, you will spend your time on the beach and in local restaurants and cafes so dress accordingly, pack light and leave space for your shopping!