This excellent four- to five-hour hike or mule ride is southeast of Agua Blanca. The trail ascends a transition zone to humid remnant cloud forest at San Sebastián, about 600m above sea level. Mules can be hired if you don’t want to hike the 20km round trip, and guides ($20) are mandatory, as are socks, long pants and mud boots (boots provided here). Bring a bathing suit to cool off in the small waterfall. Also a good area for spotting birds and monkeys.
Contract a guide in Puerto López or Agua Blanca (the rangers should be able to help you book) or ask in San Sebastián. Overnight camping by prior arrangement.