As your fellow countryman, Lifey, points out, you've got enough time to cover a good chunk of the United States, and there simply is no one-size-fits-all "best" route. But very generally speaking (and showing my own prejudices) I'd have you start by looking at I-40 across the Southwest (Grand Canyon, Petrified Forest, Petroglyph Monument, Cadillac Ranch) to Memphis; US-78/US-280/I-75 to Orlando and other spots in Florida (Cape Canaveral, Daytona Beach, St. Augustine) and then up the coastal route (Okefenokee Swamp, Charleston, Outer Banks, Chesapeake Bay) to New York; and finally I-80/I-76/I-70/I-15 back to Los Angeles (Cuyahoga Valley, Chicago, Oregon Trail, Rocky Mountain National Park, Arches, Bryce Canyon and Zion). There is plenty more to see along that general route, and other routes that may be more to your liking, but that's certainly one place to start.

AZBuck