Krabak

Taksin Maharat National park is not commonly known for it's waterfalls. There's the tree which it's claimed to be Thailand's largest (of it's sort; 16m in diameter, 50m high and 700 years old) and somewhere in the park is also a natural rock bridge.

Visiting the tree known as Krabak tree (Anisoptera Spp., an endangered tree species) though has the added value of also visiting the Pang Ah Noi waterfall, which is situated a km downstream.
The fall is rather non-descript though, simply some of water crashing 20m along a rock face. That said there are a few nice pools (along the stream not necessarily under the fall), but the jungle here is pretty intense. On our visit (October 2010) leeches (literally) were out in full force and though they're not the kind of creatures to permanently damage you, the remainder of the day was very bloody. In all the trek to the tree and waterfall is a nice 1,5 km triangular trek, not too complicated or strenuous, but rewarding and devoid of people.

Pang Ah Noi, with the emphasis on 'noi' (Thai for small)

Taksin Maharat National Park is located in Tak province, lower northern Thailand and the park was initially named after the tree until someone saw the light. There's this description:

'Taksin Maharat is a rugged mountainous park, often swathed in cooling mists. Thanon Thongchai mountain is a major watershed area. Evergreen forest and pine forest cover the upper hills, with deciduous and dipterocarp forest in the lower elevations. Wildlife includes serow, sambar deer, barking deer, golden cat, wild pig, and bear. Visitors can enjoy cool fresh air all year round. The nights can be chilly during the winter in the months of November till February'.

The park (and tree and (possible other?) waterfalls) are easily accessed from the Tak - Mae Sot highway 105, the national park entrance being 26 km up (elevation 1000m) the road from Tak.
Possible to combine with Lan Sang National Park which is nearer Tak town. The entrance fee to both is the standard fee for entrance with the possibility of getting 2 (National parks) for the price of 1!

The Krabak tree after which the national park was initially named.

Elsewhere on the internet is another visit blogged here (more on the natural bridge) and here (for birders).