Cutting through the Crocker Range National Park is part of a century-old trail used by the Murut, who carried jungle goods through the mountains and down to the coast to trade for salt. The Salt Trail is 34km long and is Sabah's most demanding trek, complete with chest-high river crossings and navigation through primary forest. A guide is absolutely essential.
There are two KK-based operators that offer this trek: TYK Adventure Tours and Top Peak. Treks tend to be five days and four nights, going from Tikolod to Inobong via the indigenous villages of Kionop, Buayan and Terian. The guided treks cost from RM1200 per person and require considerable physical fitness. Expect numerous steep ascents and descents (the highest and lowest points of the hike are 1320m and 220m, respectively), wildlife-spotting opportunities, mud, leeches and a tremendous sense of achievement. You'll be overnighting in basic accommodation in the villages along the way; bring own sleeping bag and sleeping mat and make sure your rucksack is waterproof.