Nigel, I'd take 287 to Dumas, then 385 through Dalhart and Clayton, to I-25. Then at Trinidad, I'd take CO-12 around to US-160, east to Walsenburg, then CO-69 through Westcliffe, east on US-50 to CO-9, north on 9 to Kremmling, east on US-40 to CO-125, through Walden to CO-127 which turns into WY-230. Take 230 to Laramie, then hop back on good ol' US-287 till you get to WY-34, take it to I-25 to Wheatland, then Grayrocks Rd (WY-160) to South Guernsey Rd. MS Streets and Trips shows the Fort Laramie road as paved, but the last time I was there it wasn't. South Guernsey Rd, takes you across the Oregon Trail just south of Guernsey, and you can see Register Cliff, a National Historic Landmark where you can see people like Jim Bridger's signatures.
Then just east of Guernsey, I'd take WY-270 and follow it up to US-18, go east on 18 through Lusk and then on east of Lusk on US-20 through Harrison, NE, past Fort Robinson (where Crazy Horse was killed), then north out of Crawford on NE-2/71. It takes you through Ardmore, SD. If you've got a GPS, it goes blank north of Ardmore, but 71 continues as a nicely paved road all the way to Hot Springs, SD. Out of Hot Springs, you'll be in Wind Cave NP (watch out for roaming bison, BTW), then past Pringle, into Custer, past the Crazy Horse monument into Deadwood, Lead, and then down Spearfish Canyon to Spearfish and the world's best city campground.
Then you can blast back on I-90 to US-83, down to North Platte, NE, then I-80 to York, then south on US-81 to Salina. From there you ought to be able find your way back.
It would be quite a few miles, but man, you'd see some of the best country in Colorado and good country in eastern Wyoming.