On the western edge of Khao Yai forest, not anywhere near the main checkpoints/visitor centre, there are a few trails and waterfalls which are in close proximity to Bangkok. You can be in the jungle within two hours, by car.
Chet Kod Nature Study area is more popular than the waterfall of the same name. It is basically a large campsite around a picturesque lake. There can be almost as many tents here as the main campsite in Khao Yai. There is a dirt road trail between the lake and the waterfall, and also a less maintained trail along the stream. The lake itself also has a circular trail around it. If not daytripping, the entrance gate closes at 8pm, after which you would have to drive to the waterfall area to camp for the night. From the waterfall, there are some unmarked trails heading into the centre of Khao Yai jungle - definitely man-made to begin with but after a while it becomes hard to distinguish between the trail and animal tracks. There is elephant activity as well as annoying horseflies that only seem to live where large mammals are present. You can also hear gibbons more often as you start to venture into the centre of the forest.
I wonder if this is the start point for a trek to the centre of Khao Yai. I have read about multi-day treks starting from this north western edge e.g.
www.ourweb.info/01/photos/thailand/021/index.shtml but almost certainly the path will not be maintained and you would need a ranger with machete and other gear to lead the way.
edit: Javan caecilian in the rain on the road near the camping area.
Krok I Dok Waterfall is my kind of trail because there's nobody around and it's quite a long walk, about 4-5km each way. When you arrive at the end of the road at the start of the trail there is just a sign pointing you in the direction of the waterfall, and a block of derelict toilets. You expect more when you arrive because it is quite heavily signposted on the way.
this is the end of the road...
Despite the lack of infrastructure, the trail is fairly obvious. I think enough people are walking this year round to keep the trail passable. If in doubt, stay close to the river. I don't remember being more than 50m or so from the river course at any time. The trail is not really properly marked but there are some plastic direction markers now and then at things like stream crossings.
I think the water flow might be disappointing in dry season because even in rainy season it was not spectacular, but the walk through the jungle is the main draw imo.
view from the main waterfall looking back towards where you just walked
Here is the GPS track for this trek: