I don't like Phuket much because the development and tourism has got out of hand, like Samui. But it has some saving graces. E.g. the off-the-beach snorkelling is very good and there are still deserted (and pretty) beaches if you know where to go.
There is a patch of forest in the north east of the island which has surprising biodiversity, considering the sprawling development that makes up most of the rest of the island. Phuket has two national parks but the other one (Sirinat NP) is basically just a very long beach adjacent to the airport - it's the place where everybody gets selfies at the perimeter fence with the planes 100m over their head.
In this jungle there are two main waterfalls, one either side of the NP. They're called Bang Pae and Ton Sai and there is a 4.5km trail between them which cuts right through the centre. In dry season the waterfalls are not really worth bothering with (even without considering the 200 baht fee per foreign adult) but in rainy season there is decent flow. This trail reminds me of a trail on Koh Chang which runs through the heart of that island between two waterfalls (Klong Phlu and Than Mayom). But those waterfalls are much bigger and the trail longer and harder to follow.
It's not difficult to locate the trailhead at either end - it starts right next to the waterfalls, however it's easier to start from the Bang Pae side if you have a choice because the trail at the start of the Ton Sai side is not as obvious and cuts across the riverbed a couple of times. There is a 'do not enter without guide/permission' sign whichever end you start from...
I'm not sure I would describe this jungle as untouched, there are quite a few rubber trees scattered around which I guess is encroachment from before this was officially a national park. I read that they mix the rubber amongst the jungle so it can't be distinguished on satellite photos from above. You don't get the feeling that around the next corner you're going to bump into a bear, but there are gibbons here, you will likely hear them. I saw a hornbill near the Bang Pae entrance, too.
Bang Pae Waterfall also has a 'Gibbon Rehabilitation Project' (I imagine they have a history of substance abuse and are in therapy getting their life back on track) and you can see these just off the waterfall trail.
www.gibbonproject.org/
About half-way into the trail I found a foreigner who was lost, he was wearing beach sandals and shorts and nothing else. Anywhere else and you might be surprised but in Phuket it's par for the course. I think he'd taken the trail expecting it to loop around. I asked him which end he'd come from but he didn't know the names of anywhere. He said he'd come by motorbike so I was trying to get him back to his bike, in the end we just agreed to carry on the way I was going. It turned out to be the other end to where he had started from and I believe he asked the rangers there for assistance/a lift back to the Ton Sai side because when I got back to Ton Sai the rangers asked me if he was my friend. Also his sandles broke with about 1km still to go so he was barefoot for the last stretch. This trail is sort of marked and he was on trail, so he wouldn't have died or anything but he seemed relieved when I showed him the GPS track I was following.
As with pretty much any jungle trail it's recommended to have the GPS track if you are doing it without a guide. The track can be found on the map on the free version of Backcountry Navigator app. Just zoom in on the general area and look for the dotted line. Or this is my trail which you can download and open in any trail application:
I saw two pit vipers on this trail. Pretty sure that this is the Phuket Pit Viper (Trimeresurus phuketensis):