We did a 3-day trek here to 'The Pinnacles'. Luckily the first day was an easy flat walk, because we still had 'bambi-legs' from the descent of Kinabalu. The second day was a walk of 2.4km distance on a map - but it climbed 1.2km in that distance! This was brilliant fun, a lot of the climb was up fixed ropes and ladders and felt almost like rock-climbing. The view from the top looked across to the limestone rock formation of the Pinnacles, and across the miles of rainforest in every direction. On the trek, we slept in a small camp on a wooden platform with a roof over us, but otherwise open to the elements.
Mulu also has some amazing caves, some had some incredible stalagmite and stalactite formations, others were just vast open spaces; one of them was 180m high in places and was home to over 3 million bats. we went there in the late afternoon, and as we were leaving, they all flooded out of the cave for the evening which was an impressive sight.
Mulu is a large UNESCO-protected national park of virgin rainforest, which borders Brunei, where they don't need to cut down their forest for economic reasons. This means it is a large enough area of forest to create its own weather systems: the rain falls in the afternoon and evening so heavily that the tree canopy is dripping and the the ground is flooded and then the morning sun causes it to evaporate, creating an afternoon thunderstorm. Add in 2 monsoons and it gets 6 metres of rain a year! More notable than the rainstorms was the all-encompassing dampness even when it wasn't raining. We've got used to it being so humid that you spend the entire time soaked in sweat, but here you would hang your clothes up in the room at night and in the morning they would still be just as damp as when you hung them up. After sitting outside for half an hour with a notebook, the paper became so damp that you couldn't write on it any more.