The last cave along this stretch of Rte 12 is the touristy Tham Nang Aen, about 18km from Tha Khaek. It's well lit inside, but a little kitsch in terms of the colour scheme. It is also possible to take a small boat deeper into the cave via an underground river.
The turn-off to the cave is indicated by a clear sign just past a left-hand bend 16km from the junction with Rte 13. The 700m-long track should be passable at all but the wettest times.