It is never very cold in Kona. Shorts and capris are year around wear. You may need a light jacket if you are out on the water though.
Take along some comfy walking shoes for walking up and down Alii drive.
Of course a swimsuit is a must.
Photo Equipment: The sun is very bright so you would do well to have a sun screen for your camera. If you don't have one try not to take pictures at midday.
Miscellaneous: Take plenty of sunscreen. It is expensive in the shops.
A jacket will be needed if you plan to go to the volcanoe, otherwise, short sleeves and shorts (and swim suits) are fine.
Toiletries and Medical Supplies: Lots of waterproof Bullfrog sunscreen.
Photo Equipment: A 35 mm camera, a digital video camera, a digital still camera, and an underwater camera----at least if you're a photo hound like I am.
Miscellaneous: Snorkel gear!