When you mentioned that it seems all the good places to visit are past Colorado, you probably meant the popular National Parks. Well here are some ideas of other things to see that you may find interesting, too.
Places I've been or attractions I've seen that were interesting, to me at least:
Dodge City, Kansas - Front Street Museum and other attractions
Mount Rushmore, South Dakota
Wind Cave National Park, South Dakota
The 1880s Train, Keystone, South Dakota
Cheyenne, Wyoming - UP Railroad yard and steam locomotive shop, Lions Park (home of museums and the Cheyenne Botanic Garden), historic Downtown and Capitol
Guernsey, Wyoming - Oregon Trail Ruts and Register Cliff, Guernsey Reservoir
Branson, Missouri - Silver Dollar City, Table Rock Lake, numerous music theaters
Scottsbluff, Nebraska - Scottsbluff monument and nearby Chimney Rock, both historic sites along Oregon Trail
Sydney, Nebraska - Cabela's store
Other places I'd still like to see that are close to the places mentioned above:
Hutchinson, Kansas - air and space museum
North Platte, Nebraska - U.P. Bailey Yard, the world's largest rail yard
Wall, South Dakota - Wall Drug Store and Badlands National Park
Other suggestions for information about places to see:
Check the National Park Website for suggestions:
U.S. National Park Service - Experience Your America
Check State Park websites for the states you may travel through
Nashville and Memphis, TN - Lots of music history
Civil War sites
Various museums associated with industry, such as coal mining in West Virginia, cattle drives in Kansas and Texas, or the railroad and steel industries in Pennsylvania
State and city tourism websites for those places you may visit