Is distance a factor? The Colorado below Glen Canyon Dam is one hell of a float. You can take the back haul all of the way to the dam and float down in 2 days. It is a flat water trip the whole way down.
You also have the back haul option on the Gunnison when it exits the canyon. Easy float, nothing too serious rapids wise.
Don't forget the Green River below Flaming Gorge Dam. A section is super simple. B section is easy too (even with Red Creek raging). C section you sometimes have to actually paddle to go DOWN river