This was a great trip - just a few thoughts - some obvious - hopefully others may be useful.....
Camera lens wipes - if you have a compact or bridge camera you probably won't have a filter over the lens - if you use a tissue or the inside of your T shirt to clear the lens of humidity, sweaty/greasy finger prints, dust etc you may scratch the lens. So take enough sealed moist lens wipes for at least the "jungle" part of the trips. Don't have to be specialist - I took Tesco Lens Wipes.
Batteries - there will be the opportunity to recharge your camera batteries at the hotels/lodges BUT with compact/bridge cameras the constant zooming in and out, along with the effect of humidity/temperature, means the battery will not last as long as normal. I always take at least a couple of spare charged batteries out with me - and when you are told you can leave your bags etc in the coach, eg for a photo opportunity, don't forget to pop a spare battery in your pocket! Constantly reviewing what you have taken will drain the battery too. If your camera "freezes" switch off and on again.
Binoculars - beg, borrow or buy some good bird watching binoculars (don't have to be big, just good optics) and take time to practice using them before you leave home - it makes searching the tree tops much easier and very rewarding!
Do take time just to enjoy the experience of being out in the jungle/on the river surrounded by the sights and sounds.
Torch - on the night walk we were asked to only shine our torches on the boardwalk - so no need for more than a normal small torch which would also be useful if the lights failed in your room.
In the lodge rooms there are not many electric points and one is used for the mosquito plug in tablet each evening. I took a two way adaptor so I could use the plug in and charge my camera battery at the same time. (All sockets took typical UK 3 pin plugs)
Ants - I have a habit of sitting with both elbows/arms leaning on the table or when standing, leaning against the wall or on posts/rails etc. Just be aware that insects, ants/red ants may be making their way up, down and along and will take umbrage at being squashed - I ended up with no mossie bites at all but a neat collection of itchy ant bites/stings on my arms!
Clothes - the combination of DEET, sun creme and sweat! can stain clothing (especially whites). You don't need an extensive wardrobe in the jungle. Most trips are done in a boat - consider whether you need to cover your shoulders against the sun, wear a hat. Males and females wore (longish) shorts, crops or long trousers on the different trips, with some ladies opting for sleeveless tops, otherwise short sleeved tops and shirts, some wore long sleeves in the evenings or used light shawls. Lightweight, natural, quick drying materials are preferable. We were luckily not greatly bothered by insects in the evenings but the general advice is to spray insect repellant and cover up after dusk.
Footwear - The itinerary suggested walking boots. In reality I wore my proper walking boots once, on the first rainforest walk where the ground was a bit uneven in places. I also had normal trainers with a good grippy sole and wore these or well soled walking sandals on all other walks. In my opinion the trainers would have done for the first walk too. On the boats and around the lodges sandals were fine - boardwalks could be a little slippy if wet or in the mornings
Rain gear - it did rain on and off but mostly after we had completed our trips. We had rain during one of our river expeditions but it was still very warm - most people wore very thin ponchos - they are easily available in Borneo - I bought mine at the Sepilok Orang Utan centre and it cost 5 Ringgits - about £1. Great fun trying to pull them on and more than one of us ended up with the hood at the front, even at the second go! Worth thinking about a Dry Bag to pop your camera into. In Singapore I would suggest do as the locals do and just use an umbrella - I used mine as both sunshade and rain protector - I had bought a silver collapsible brolly to take to Singapore 5 years ago and used it this time too.
Longer hair! Don't forget clips, bands or scrunchies - much cooler with hair tied up off shoulders and back.
Swollen ankles - even some people who do not usually get swollen ankles ended up with puffy ankles and feet in the heat and humidity. Apart from the usual advice to drink plenty and try and put your feet up when resting, it seems this is just one of those things. If you have strappy sandals or laced shoes you need to be able to loosen them - some of us ended up in flip flop, fit flop, minimal sandals. Several of us (male and female) had foot/lower leg massages during our stay in Kota Kinabalu with differing results - be aware in the hotel spa they throw in a vigorous shoulder and back massage as well. I have used Frio ankle bands in the past - same cooling theory as my insulin transport bags. Also wrapped my ankles and feet in cold wet hand towels! Good news is once back in the UK summer my ankles immediately returned to normal!!!!!!!
Dehydration - it was hot and humid - but we all managed reasonably well. I got used to literally dripping sweat! The hotels and lodges supplied 2 free bottles of water each day and water was always available on the coaches and some boat journeys. Both lodges had tea and coffee available all day and evening, plus drinks to purchase. Many people took Diarolyte or similar rehydration sachets daily. I understand it helps to sip water at short intervals as well as drinking plenty of water, tea etc. I now have the taste for both Sabah tea and green tea. The gift shop at the tea plantation had lots of types of local tea - make great souvenir presents. Our tour manager in Sri Lanka last year advised drinking Coca Cola (original not diet) as this has a good balance of sugar and salts - it seemed to do the trick - my doctor was horrified!
Remember to take a small tube of sun lotion on the day/boat trips - especially to re-apply on your face - and nose in particular!
Finally - I had the most wonderful time in both Sabah and Singapore. I trust you will enjoy this amazing holiday!