I wimped out a little on this one, and took an organised day trip from Bangkok's tourist enclave of Khao San Road. However I have to say the trip was pretty worthwhile, but I would have preferred to spend more time in the Park proper and perhaps got a little more chance to hike around and try to glimpse some wildlife.
After some stops for general touristy things like visiting a market, we entered the park and headed straight for it's highest waterfall at Haew Narok which provided a nice glimpse of the environment. Our guide didn't seem to concerned on keeping noise down to allow a better glimpse of some wildlife, and when we did finally see a chameleon up close the other members of the group seemed a little disinterested.
We then drove through the park and spotted some deer by the road side before stopping at a view point where there was a large group of macaques.
I was impressed by the infrastructure of the park, I really reminded me of the national parks in US that I have visited, except the food stalls sold eminently more tasty looking food. This reflected a lot of what I found in Thailand, it is a rapidly developing country and amenities for tourists are very impressive.
After we left the park we went for a meal at a 'Jungle Lodge' just outside the park proper. We then got to ride Elephants, I must admit I was not really holding out much hope of it being anything more than time filling, however I really enjoyed my brief half hour trip, and it added something extra to the day.
All together this was a nice and easy trip to make from Bangkok. If I was to do it again I would certainly try and spend a little more time in the park, and especially do a more substantial hike. However what I saw of then park was impressive; and I guess that the diversity of wildlife would be very rewarding for those with a keen interest in it.