Tham Nam Lod
Guides (with lanterns) for Tham Lod are only 150 Baht per group (1-3 people), and the bamboo rafts inside the cave are 400 Baht for 3 people (return trip through the cave). It takes about 2 hours to see all three higher level caverns, and the main river tunnel. You can watch 300,000 birds fly into the cave every evening after sunset, in the twilight, for free.
Tham Lod is an enormous river tunnel with 3 higher dry caverns. The cave walls are covered with massive formations and columns over 20 metres high tower over visitors. The third cavern shelters the remains of prehistoric teak coffins. Near the visitor centre is a recently excavated archaeological site- a rock shelter where a 20,000-year-old human skeleton was found- the oldest in Nth Thailand. Info on the site is in Thai and English. Sometimes wild gibbons inhabit the forest near the cave entrance- follow the whoops and you may see them.
During the wetter months (around July to December, depending on river levels) Cave Lodge runs fun kayaking trips that go through Tham Lod, then continue downstream for 6-12 km. On some trips, you can visit wild (undeveloped) caves along the way.
Kayaking trips are 650-1200 baht/person and can run any time, any number of people. If you are less confident you can sit with a guide. When the river is particularly high, everyone except experienced whitewater kayakers sits with a guide.
Tham Lod is a very photogenic cave so bring your camera!