Sorry Gwamma, Zoe is correct - Minnesota.
Just looked up a couple of things. You can get to Yellowstone, Mount Rushmore, and the SPAM Museum (LOL) using I90 (Interstate 90) out of Seattle.
Just a note, though you guys are vacationing and I was moving. When I moved from NJ (New Jersey) to WA (Washington state), we wanted to drive through Yellowstone. It's so vast! We'd drive what seemed like hours, and I'd look on the map and it seemed like we'd gone almost nowhere. Finally (but remember, I was moving and was in the front of a small pickup with my ex and a sort of big old dog), one of us said, "F*** it, let's get to I90." *chuckle*
Google maps is pretty good at plotting entire trips, or even portions if you decide to go off track to see something. I found it invaluable during my last move.
Cheap cell phones are readily available at Walmart. I don't use one in everyday life, but for the road, I picked one up for maybe $20, and the minutes were (I think) $30 for 100 talking and unlimited texting. Not great, but cheap for emergencies. Also, almost all motels have wifi for free or cheap, so once you're in the motel, you can skype or google phone, or whatever freebie is available to you. Assuming you're bringing your laptop.