Not for the faint of heart or those fearful of exposed trails. Starting from Saddle Mountain trailhead (Highway 67/FR 610/Trail #57 at 8,800 ft) run down the Nankoweap Trail to the Colorado River, and back. After leaving the rim, the Nankoweap Trail snakes along the red esplanade sandstone. The trail is fairly well marked and cairned until Nankoweap Creek, where it eventually becomes a sandy boulder/scree field that meanders down the creek until the confluence. Highly recommended to cache fluids along the way and start well before sunrise, as temperatures in the depths of the canyon regularly rise to over 100 degrees. Below is good beta from NPS. It is dangerous and exposed and there is little room for error considering it is not easy to get a signal, but the views, the solitude, and the call of the canyon are extraordinary. Nankoweap is the largest rim to river drop of any trail in Grand Canyon, plus once you are out of the canyon, you still have two significant ascents to reach the Saddle Mountain trailhead.
Note you may want to visit the famous Nankoweap Granaries, but that will of course add to your TH-TH time.
I never thought I wanted to do a route into the Grand Canyon until just now.