Lee's Ferry is the official beginning point of the Grand Canyon. It is also the only place within hundreds of miles that you can drive to the Colorado river so it makes it a natural launch point for Colorado rafting trips. It is about 16 miles downriver from Glen Canyon dam. This 16 miles upstream is basically flatwater and makes for a great, albeit long, day trip on the kayaks. We got a backhaul up river and paddled back down.
There are also numerous canyons or "washes" that lead down to the river at various points. These were fascinating in that they allowed one to walk down through the first several layers of the Grand Canyon strata and see it up close.