Havasu Creek tumbles through the Grand Canyon on its way to the Colorado River, creating a series of waterfalls and swimming pools about 10 miles below the rim on the Havasupai Reservation. Note that rain and floods can affect the flow of and access to the falls, and getting to them can be tricky; the only access is via foot, mule or helicopter, and trails can involve multiple river-crossings and ropes.
Check the website for details on accommodation, current conditions and tours. There is only one lodge in Supai, 0.75 miles up from the first waterfall, and one campground. Reservations for the lodge are accepted by phone only (beginning in June for the following year); reservations for campsites are online only (beginning in February for the same year). The trailhead is located four hours from the Grand Canyon South Rim.