The tropical days are hot and humid and we recommend light, comfortable cotton shirts. Casual wear is acceptable for most occasions in the city; on trips to the interior, shorts, and T-shirts may be more appropriate. Thin or lightweight, long-sleeved shirts and long trousers will protect you from mosquitos in the evenings.
In terms of personal toiletries, apart from your standard cares (such as shampoo, soap, toothpaste) and personal medication, bring along insect repellent and a basic first aid kit that includes band-aids, mild antiseptic cream for insect bites or light scratches, cotton swabs, tissues, bandages, and painkillers.
Also, remember to pack an appropriate travel (plug) adapter for your electronic devices (see below for the type of socket for Borneo). For most day trips, it is a good idea to bring along a refillable water bottle and a foldable daypack that can be put in the transfer vehicle.
On some trips, you can also bring swimming costumes, as there are chances to swim in a river or take a dip in a waterfall. However, if you are visiting the mosques or temples in Borneo, it is best to cover yourself fully to show respect, including a scarf to cover your hair for females.
A torchlight or flashlight is always useful if you go into a cave chamber or stay overnight in remote villages. An additional consideration for the longer overnight trips is to bring along overnight bags or rucksacks rather than hard luggage, as it might involve carrying in and out of boats, or multiple steps up longhouses, and trekking in the jungle.