As famous now for its recent collapse as anything, Kuangsi was a tower of champagne-glass limestone formations until the whole structure fell in on itself in 2003. Locals say that tour operators became too greedy and neglected local spirits, called Pi. The falls are still beautiful, but less so. The ride here, however, is quite spectacular. You'll have to travel by songthaeaw (covered pickup) for $5 per person if shared, or by boat and tuk-tuk for the same fee.
Another option, Tad Se Waterfall, is 21km (13 miles) from town and good for swimming, even if it's less spectacular in height than Kuangsi. During the rainy season, the falls are stunning. Hire a driver for about $5 or pay a bit extra for a ferryboat.