A few kilometres from Kaeng Krachan's main entrance gate, is a private garden and guesthouse called 'Ban Song Nok' ( there is a fork in the road about one kilometre before the entrance gate - take the right-hand fork and follow for about 4km until you see the sign for 'Ban Song Nok' on the left). The owners have setup an excellent blind to watch birds come in to drink and bathe at a small man-made waterhole. As the site is popular with birders, you might have to make a booking as they only allow a few people at any one time to minimize disturbance. They charge 200THB per person and I think you can stay as long as you want, although the first and last two hours of the day are usually best for birds. Make sure you are well-prepared for mosquitoes!
The small waterhole is the main attraction for birds here, so it's generally better in times of dry weather. I visited late one afternoon in December 2014 and saw about 15 species in less than two hours but I'm sure on some days you could see almost double that.
Interesting species that can be seen here include: Kalij Pheasant, Bar-backed Partridge, Scaly-breasted Partridge, Slaty-legged Crake, Red-legged Crake,
Lesser Necklaced Laughingthrush, Greater Necklaced Laughingthrush, Orange-headed Thrush, and Siberian Blue Robin.
Here are some links with more details on this site -
If you're hoping to photograph the birds, make sure you take a tripod and your camera can handle high iso (3200+) as it can be quite dark around the waterhole when the birds are active early and late in the day.
I shot these at ISO6400 on a Canon 6D with a Tamron 150-600 f6.3 lens.