I like Shifty's idea of just taking the train here to Flag. In fact you could also take the train from Williams directly to the South Rim and then ride back. Riding from Williams will only allow you to be in the pines for a short time before it opens up to full desert exposure. If instead you ride from Flagstaff out Hwy 180, you get a pretty ride for about 35+ miles of rolling hills passing between Kendrick Peak and the San Francisco Peaks and through aspen and pine forests. In addition to hot temps in the desert during the day, also be ready for cool nights with temps in the 50's. Also beware of the summer monsoons. They are a daily occurance. I got caught in a hail storm today during a high elevation ride just north of Flag. The Canyon gets it too. I would stay away from the Cameron side - very boring and full desert exposure. If you are feeling very adventureous you can actually ride all the way from Flag to the Rim via the original dirt stagecoach road used in the 1890's to buggy tourists up to the Canyon from the BNSF rail station in Flag. There are still busted wagon wheels along the road and historic sign posts. If you choose asphalt, auto traffic is usually light but they motor at high speeds at 70+ mph. The roads are two lane with no shoulder for most of the way. Be careful! Hope you have a great time!