I would like to make a trip to the Great Plains - specifically, Montana and the Dakotas - because I haven't ever been to that part of the country, but I know nothing about logistics of getting around once I'm there. Renting a car seems like the option that would give me the most flexibility, but it would be expensive and there's also a LOT of miles that those states cover. I don't have specific plans yet, but this would probably be a one to two week trip in summer (no snow!). I don't mind long car / train / bus rides, but I don't want to spend all my time in a car and I don't want to spend a fortune.
What are my options for traveling within and between those states? Is renting a car the best way to do it, or are there public or private transportation options that I should consider?
Note: I'd be flying out there from the east coast, and my question is not about how to get there, just how to get around once I'm already in the region.