I know little about Kansas, Colorado, Wyoming, South Dakota, and Montana but I could point you toward all sorts of ghost towns you could poke around here in Nevada. Private property in these here parts is most likely posted as such and a good map showing what is public land (Poking about allowed) or private is essential. Active mining operations may occur on public land so a healthy respect for posted no trespassing signs is sage advice. Many of those sites can still be photographed from the road or the street. Hopefully someone with a good knowledge of the states you plan to visit will respond.
I have found doing the research before visitng can sometimes open doors. Imagine a conversation with a rancher that goes like this. Howdy, just admiring the old house near your stock water. I understand that land was first settled with the mine in 1897 but that the Hackett Family worked this place as a ranch until the late 1950s.
The crusty old cattleman replies Yeah, you know that family is buried up that way.
You reply "Yes, I know that, I try to swing by here every Memorial weekend to pluck weeds, clean up the site.
A Note: Two of the Hackett sons were killed in WWII and their military style headstones explain why there are no future generations of Hacketts. The family gravesite is not on the private land the cattleman now owns.
The cattleman, now assured you are genuine replies Yeah, do you know about the root cellar up behind house, I found some of the boys toys up by it a while ago. He replies that he has to be going and tells me feel free root around but please close the gates when I leave.
Have fun this summer.