There's a reason they call it the trip of a lifetime. Rafting the Colorado River is truly awesome, not only for its monster rapids, but also for the ever-changing beauty of canyon walls and the solitude of sleeping under the stars in the inner-gorge wilderness.

So you've scored a river permit, gathered your buddies and hired an outfitter to supply your crew with cookstoves and ammo cans. Or maybe you've booked a week on a commercial trip and can leave the planning to a pro. Either way, you're about to experience one of the most mind-blowing rivers you'll ever have the privilege of running. All told, the layers of canyon geology through which you'll travel represent at least 2 billion years, and if that doesn't get your rocks off, the whitewater will. With so many notable rapids, beautiful side hikes, camping beaches and put-in and take-out points, this trip's route should be viewed as a necessarily exclusive selection of highlights.

Get ready to head down 87, 188 or maybe all 279 river miles of the mighty Colorado. Take a virgin dip into the frigid river at Lees Ferry, and then get in. Take a look at the low-lying cliffs of the Moenkopi formation atop darker red Kaibab limestone rising around you, the top layers of Grand Canyon strata. Then glide onto the Colorado and muse on the depth -- literal and figurative -- of this venerable canyon you're rafting. As you gently float away from Lees Ferry, you'll enter Grand Canyon National Park after several miles of smooth water and riffles. But after some fun first-day rapids like Badger Creek and Soap Creek, you'll get the hang of the Colorado River rating system, which classifies rapids from Class 1 to 10 (rather than the standard I to V).

On your second day, stop at North Canyon for the hike through this side canyon leading to a small, seashell-like grotto. The curves, ridges and slabs of this side canyon give a tantalizing first taste of what you'll explore on stops along the river. Once you return to the river, you'll quickly bounce into the Roaring Twenties, a series of smallish but not insignificant rapids along mile markers 20 through 29.

Day three you'll drift by the verdant cliffside oasis of Vasey's Paradise, kept green by the water springing directly out of the wall, before rounding a bend and spying the wide mouth of Redwall Cavern. As you approach, the scale of this enormous cave will surprise you.

Though the fourth day brings hours of drifting between a few good rapids, highlights include a hike up to ancient Puebloan granaries at Nankoweap. Downriver, marvel at the intensely saturated purple, blue and green layers of Bright Angel Shale before stopping at the confluence of the incongruously warm, turquoise-hued Little Colorado River.

Big-time whitewater is on tap for day five, with monster rapids like Unkar, Nevills, Hance (rated as Class 7 or 8), Sockdolager and Grapevine socking you with some of that cold Colorado water and a fat adrenaline rush. This is what it's about. Between exhilarating drops, catch your breath and check out the oldest rock layer in the canyon, which now appears at river level. The smooth, black Vishnu schist, shot through with pink Zoroaster granite, is some of the oldest exposed rock on the planet, and it marks your arrival to Phantom Ranch.

At the end of your six- to nine-hour haul up the Bright Angel Trail, reward yourself with an ice cream at the South Rim's Bright Angel Fountain. After your (heavenly) shower, sup on half a citrus-glazed roast duck, sip a prickly-pear margarita on the back deck and lie down for a very sound sleep at the historic El Tovar.

If, lucky you, your river time flows on, you'll continue floating under the suspension bridges near Phantom Ranch to hit several serious rapids, beginning with Horn Creek, with the challenge of Granite soon thereafter, and finishing with the famously burly Hermit Rapid. After punching through the waves and holes of these beasts, you'll be elated, exhausted and ready to spend a calm night between the soaring schist walls of the Upper Granite Gorge.