I think a Sandal with Full Strap is a must!!! why? because u can wear it while tubing and river swing! if u don't wear it, then u have to risk ur feet to step on something unnecessary. if you wear slippers, you might lose it!
For Ladies: bikini for the water games and a big scarf to cover your body while visiting villages
Toiletries and Medical Supplies: Sun Block, Mosquito Repellent
Photo Equipment: Water Proof Camera..
Exploring the cave need a pair of good shoes. Dont wear slipper because the cave can be slippery.
Miscellaneous: Torchlight is a must when you go to Vang Viang. One of the reason is stated in warning and dangerous tipr- there is no street light at night. The other reason is if you want to check out some of the caves, torch light is definitely a must.
Bring a good torch and enough spare batteries into the cave. You wont know how many time you will spend in an unexplored cave..