I have spent a fair amount of time in several places along your proposed route. A couple of suggestions : While in Denver, a visit to the Colorado Railroad Museum in Golden is well worth the time. From downtown Denver (Caboose Hobbies on Broadway) you would take "6th avenue" (it is a freeway) West to Golden. It is located close to Coors on the NW side of TableMesa. Lots of things to see there, including at least 3 RGS railcars (2 Galloping Geese and one MOW). From there you can continue on 6 up Clear Creek canyon into the mountains. Great scenery with lots of tunnels on that road, just watch out for drunks and tourbuses coming back from Central City. CC and Blackhawk are still interesting if you are interested in mining(or gambling or cheap Prime Rib dinners), although that place is mostly a gambling tourist trap now. This route is a lot more interesting than taking I-70 up to Georgetown. If you stay on 6 you eventually cut over to I-70, at which point you will see a little restaurant called "Kermits", which is a good place to eat if it is still there. From there it is maybe another 15 minutes on I-70 to Georgetown. I also concur about trying to visit the San Juans in the SW corner of the state. I lived in that area for several years and the scenery there is breathtaking. If you make it there "the" railroad item to experience is the Durango & Silverton NGRR. The Cumbres & Toltec is awesome too, but that area is not "on the way" to anywhere. It would actually cost you a couple days minimum to go down to the San Juans from Denver and then come back and head up to Cheyenne if that is your proposed route. I highly recommend it though if you have the time. Seeing that you are familiar with Denver you may be aware of some of these things, but if you are anything like me you may not have done the "touristy" things while you are there. I have spent a decade or so in the Denver area, including a couple years in Golden and never did do the Coors tour- for instance.
Galloping Geese in CRR museum. They gives rides on these most days in the summer.
My other suggestion is that when you are coming down the coast, you can catch a ride on the Roaring Camp RR in Fenton CA, which is in a patch of big trees just north of Santa Cruz (hour south of S.F.). You can actually park in Santa Cruz by the Boardwalk and take a train from there straight to Roaring Camp. Roaring Camp has several steam locos, including a Shay and a Heisler. The scenery there is outstanding, you are in the Redwoods and at times can look down and out over the Pacific. Be aware that there are a lot of thieves and whatnot in S.F. so park in a highly visible area (or parking garage) and watch your back and don't take shortcuts down alleys and such if you are anywhere in the North Beach, Market Street, ChinaTown area. Also, watch out for the d@mned vampires in Santa Cruz! For a more laid-back afternoon you might want to check out Sausalito on the north side of the bridge. Lot's of touristy stuff, restaurants, chatzky shops- etc., for the wife and is a lot safer than S.F.
Roaring Camp RR Shay in the big Redwoods.
Good Luck, and have fun on your roadtrip. I am stuck in the desert this summer myself, maybe next year I can get up north or out on the coast.