Sabah, Borneo

Sabah is a great place for wildlife viewing, both on land and underwater (snorkeling/diving). What to take depends on the details of your trip. It will be hot and muggy, and it could rain a lot. Umbrellas are better than raincoats/rainpants, on account of the heat. Take lots of insect repellent, and maybe spray your clothes with permethrin to protect against insect bites.
Leeches: This refers to the little black terrestrial leeches that latch onto mammals (like us) and feed on them. They are all over the rainforests/jungles in Asia, and are worst in the rainy season. They hang onto plants waiting for something to brush by, but can also sense you and head in your direction, going end-over-end. They don't carry disease, but it's kinda gross to have to pick them off. This is very different from the leeches in the Americas, which generally are only a problem if you go into the water.
In the nice resorts, they probably won't be a problem, especially if you go there in the drier season. But if you leave the landscaped areas, perhaps to visit an Orangutan Sanctuary, or do a lot of walking, or standing around looking at birds, plants etc., they could get on you. Depending on how much this worries you, these are all things we have done as countermeasures. Option1 - knee-high rubber boots with pants tucked into them, which is also good for walking through all that mud. Your resort may have boots they can loan you, but maybe only in smaller sizes. Option 2 - wear your ordinary shoes/socks with leech socks, with pants tucked into the leech socks. These are just sock-shaped garments made of tightly woven fabric such as cotton muslin. The leeches can go right through normal knit socks, but they can't go through the tighter fabrics. Option 3 - always be on the look-out for them and be ready to pull them off you.
Please ask for more info if any of this isn't clear or what you wanted to know. Happy travels!

I traveled to Malaysian Borneo in March 2012, I think. It was very hot and humid for the most part, of course. I didn’t travel to the Danum Valley, but we stayed at a resort on the Kinabatangan River where I did a rainforest walk. The resort provided gum boots, so I was able to tuck my pants into those. There were leeches on the plants and I tried not to touch anything, but I still ended up with a couple of leeches on my hand. I hate bugs, so I freaked out a bit, but the tour guide was able to take them off quickly.