This lush national park covers 958 square kilometres stretching all the way to the Burma border, though most is unreachable unless you're up for back-country trekking with a park ranger.
The main attractions are Sai Yok Yai and Sai Yok Lek waterfalls, both of which emerge as streams out of the forest before plummeting eight to 10 metres into the River Kwai. A suspension footbridge has been set up near Sai Yok Yai, which takes you across the river to a viewing point. You'll need to pay a longtail boat 300 baht to take you the half-kilometre down river to see Sai Yok Lek, oddly the slightly larger of the two (lek means "small" while yai means "big" in Thai).
A kilometre-long hiking trail near Sai Yok Yai leads to a stream up on a mountain, which is nothing more than a trickle, but it's a pleasant walk with the possibility of spotting colourful birds and other wildlife. Near Sai Yok Yai you'll find several floating rafthouse restaurants and decks where anywhere is free to put their feet in the river and watch the fish swim by, or jump in to swim with them. A handful of simple bungalows can also be rented in the park.
How to get there
The entrance to Sai Yok National Park is located right off Route 323, some 95 kilometres northwest of Kanchanaburi town and signs are well signposted if you're coming on your own. Buses to Thong Pha Phum or Sangklhaburi can drop you along the main road, from where it's a good one to two kilometres' walk to the parking lot and another half kilometre to the waterfall.