This loop takes you past three falls, and can be accessed either by the Horsetail Falls Trailhead (from which Horsetail Falls is easily visible and accessible), or the Oneonta Gorge Trailhead.
From the Horsetail Falls Trailhead, find and follow the Gorge Trail #400 (this section of trail #400 is the Historic Columbia River Highway) to the junction with the Horsetail Falls Trail #438. This will lead you first to Ponytail Falls, which you can walk behind. The trail will next take you upward with switchbacks for ~0.5 miles until you reach a bridge over Oneonta Gorge. Oneonta Falls is visible from the bridge, looking downstream. Continuing on, you will reach the junction with Trail #424. To add an extra waterfall to the trip, turn left and hike ~0.9 miles to Triple Falls. To complete the loop, turn right from Trail #438 onto Trail #424. This will lead you back to Multnomah Falls, and back along the highway to the Horsetail Falls Trailhead.