At the centre of Thong Pha Phum is a busy market, which is worth a visit, particularly for the occasional Burmese imports and local food.
The town is presided over by a glittering Mon-style chedi just east of the river and accessible via a footbridge from town at Wat Tha Kha Nun or side road off route 323. A walk up the steep hillside to visit it should fill an afternoon, and you'll be rewarded with a pleasant breeze and excellent views to as far as the dam.
One of the biggest tourist draws in the area is Vajiralongkorn Dam. Unfortunately, it's some distance from town, and not easily accessible without your own transport -- it's a six kilometre walk from town along a straight and shadeless road. You might be able to convince a motorbike taxi to take you there for 60 baht, otherwise hop on a Pilok-bound yellow songthaew from the market. It will drop you off near the dam, from where it is another kilometre hike to get there. If you can manage to make it here you'll be rewarded with some seriously stunning views -- more so of the valley to the south than the lake to the north, though that's lovely too. There's also a park with some cherry blossom trees that, along with the karst cliffs that are often shrouded in mist in the morning, lend the feel of being in the far south of China.
If you rustle together a group of people, Som Chaineuk will organise tours, but otherwise you need to rely on public transport to get around. Seventy kilometres to the west of Thong Pha Phum is the mountain border outpost of Pilok. There is not much to experience in Pilok except for remoteness and a complete lack of tourists, but it's almost worth a trip just for the stunning scenery along the windy road that runs high into the mountains (not recommended for those prone to car sickness).
North of Pilok, Thong Pha Phum National Park offers some outstanding treks on high, lush mountain ridgelines with views into Burma. However, these hikes are not exactly walks in the park -- those who conquer the 30-plus kilometre trail are rewarded with a document from park headquarters -- and the park is not easily accessed without your own wheels. If you do go by car or motorbike, be certain to have a full tank of gas and extra gasoline as you don't want to get stuck along the mountain roads. It's possible to camp in the national park, but you'll want to pack your own food and necessities. There is also a 100 baht fee to enter the park.
North and en route to Sangkhlaburi is Diachong Thong waterfall and Kraeng Kawia waterfall and a forest wat. With a number of pristine pools suitable for swimming and caves behind the falls, Kraeng Kawia is especially refreshing and it's easily accessed directly off Route 323. About 15 kilometres south of town is Hin Taat hot springs which are three kilometres off route 323 and signposted in English. The water bubbles away in a series of artificial bathing pools at around 40 degrees, though the hot springs become less enticing when you consider that the air is sometimes 40 degrees anyway.
Further to the south you will reach Sai Yok National Park with its rafthouses, floating restaurants and waterfalls along the River Khwae. There are also a couple of elephant camps: Puthong Elephant Park and Linthinland mainly cater to organised groups from Kanchanaburi but everyone is welcome. All the southbound destinations can be reached by the local bus to Kanchanaburi.