I know from experience how frustrating it can be to not only find national and or forest parks but also experiencing them. Some have no facilities at all others have great trekking routes, though mostly for an hour or two, anything beyond this, it seems that you need a guide, keeps them busy.
Anyway through the years I have quite a few experiences. My own interest is in hot springs and in waterfalls and as such I have blog sites on both. The hot spring site
Soaking in Siam
has loads of other links to scraps of info and is by far the most extensive available. The waterfall site
Waterfalls of Southeast Asia
is as it mentions wider though still I believe more than 40 first hand reports on waterfalls in Thailand.
Hopefully this will assist some and/or stimulate others to get outdoors ... There are some great places to go to and be!