I second or third the idea of driving to Laramie through Fort Collins and then I-80 to Rawlins. Turn right at Rawlins and go to Lander and stop at the Sleeping Bear Campground. Then go up to the Sinks in Sinks Canyon and over to the rise to see some of the bigger trout you will ever see. Then, the next day on to Dubois, over Togwotee Pass for that first time flew of the Tetons, down to Moran Junction, through Grand Teton National Park and on to your Destination in Yellowstone. From Lander to Moran Junction is about 2 1/2 hours so you will have plenty of time to get to Yellowstone. Great scenery and a pretty easy drive all around if you take this route.