We have visited a number of Oregon Trail sites in Nebraska and Wyoming. The link that enblethen posted is a good reference for planning.
We always get off I80 a Ogalalla, NE and take route 26 all the way to I25 in WY. Windlass Hill where the wagons were lowered down the bluff is on the left and the first site we pass. There is a short paved path to the top of the bluff with good views of the eroded slope where the wagons were lowered. The Ash Hollow visitor center is on the right a few miles further and the Ash Hollow Cemetery is a bit further on the left.
Along route 26 you pass Courthouse and Jailhouse rocks in the distance then Chimney Rock. There is a nice Museum at Chimney Rock. Scott's Bluff is a bit further on. Shortly after entering Wyoming is Fort Laramie, IMO a must stop. Several of the original buildings have been restored or reconstructed and a nice visitor center. A very interesting place.
The absolute best place to see wagon ruts is in Guernsey, WY. The ruts are cut into sandstone and in one place are 3-4 feet deep. There is a paved loop path, but it pays to walk in the ruts to find more ruts not visible from the path. About 2-3 miles away is Register Cliff with emigrant names carved into the cliff face.
After Casper, WY we take route 220 and 287 to Lander. There is a parking area at Independence Rock. The emigrants carved their names into the sides and top of the huge granite rock. Good views of the surrounding area from the top of the rock. Devils Gate is visible in the distance. Before Lander route 28 goes southwest over South Pass.
You can use Google Maps to see pictures of each of these sites that others have taken. Just search Maps for each name. Use satellite view to see each site and the surroundings.
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