With only two weeks, I would pick one or the other, take different routes going and returning and stop to see a few places along the way.
In the case of Yellowstone; along I-80, there is the
Great Platte River Archway and Museum,
Fort Kearny, and the original Cabela's in Sidney, NE which even has a campground.
If you were to take US-26 West from Ogallala, NE; there is
Scotts Bluff National Monument,
Fort Laramie National Historic Site,
Guernsey State Park,
Register Cliff and
Oregan Trail Ruts.
I-25 will take you to Casper and then US-20 will take you to Thermopolis where there is
Hot Springs State Park,
the Wyoming Dinosaur Center, and
the Legend Rock Petroglyph Site.
From Thermopolis, WY-120 will take you to Cody where there is the
Buffalo Bill Center,
Old Trail Town and the
Nightly Rodeo. A loop of the
Chief Joseph Scenic Byway and
Beartooth Highway with a stop in Red Lodge makes a nice day trip from Cody. Cody is not far from the East Entrance to Yellowstone.
Along I-90 are the
Little Bighorn Battlefield,
Devils Tower National Monument,
Mount Rushmore,
Custer State Park,
Badlands National Park and many other attractions in the Black Hills area.
A visit to Yellowstone should also include a visit to the adjacent
Grand Teton National Park.
If you were to choose the Grand Canyon, there is a lot to see not that far away. In the Flagstaff area are
Sunset Crater National Monument,
Wupatki National Monument, and
Walnut Canyon National Monument. Sedona/Camp Verde/Cottonwood are about an hour South of Flagstaff with
Montezuma Castle and Montezuma Well National Monuments,
Tuzigoot National Monument,
Palatki and Honaki Ruins,
V-Bar-V Heritage Site,
Verde Canyon Railroad,
Fort Verde State Park,
Cathedral Rock,
Gold King Mine,
Jerome State Historic Park in the area.
East of Flagstaff along I-40 are
Petrified Forest National Park,
Meteor Crater,
Homolovi Ruins State Park and
Winslow Arizona where you can stand on the corner.
