A Plethora of Possibilities
It is roughly a thousand miles from Indianapolis to the Rockies, so realistically, you're looking at spending two days on the road each way. That only leaves you four days to explore the mountains and maybe just get a taste of the Southwest, so you're going to have to be pretty selective. I agree with Michael that Tombstone is just too far and Royal Gorge can be a disappointment. But if you really want to see both the mountains and the southwest, then you're also going to have to give up some of the Utah attractions such as Dinosaur and Arches National Parks. The truth is, there is no one best plan to meet your needs, and it will be up to you and your girlfriend to decide both your travel style and the places you really want to get to. With that in mind, here is another set of possibilities. Keep in mind that both Michael's suggestions and these entail a good bit of driving and will have to be modified on the fly if the weather turns against you, and that both of these possibilities are but two of the many that are available.
Take a bit of time as you trek across I-70 to take in a sight or two in St. Louis as well as some of the quirkier stops in Kansas. Your objective is to be in Boulder by the end of the second day so that you can enjoy the Pearl Street Mall in the evening before heading out the next day to the Stanley Hotel in Estes Park and then continue on to Rocky Mountain National Park. Spend most of the day exploring the park (not enough, really, but...) and end up on the west side staying somewhere along I-70 between Empire and Vail. On your second day in the mountains, work your way south on US-24 stopping in Leadville, generally enjoying some spectacular scenery, and then heading west on I-50 through the Black Canyon of the Gunnison. This evening stay somewhere between Montrose and Durango, depending on how you do.
Your next day you'd start the southwest portion of the trip with a visit to Mesa Verde National Park, then using US-160, CO-41 and UT-162 cut over to Bluff, Utah for a drive through Monument Valley. When you come out on US-160 again at Kayenta, double back (east) to take US-191 south to Canyon de Chelly National Monument. Finish the day by heading south to I-40 and start heading east towards Albuquerque, maybe bedding down for the night around Gallup. Your last day would then entail stopping at the Petroglyph National Monument outside Albuquerque and then heading northeast up I-25 with stop in Santa Fe and/or Taos, two wonderful southwestern cities. You would then use US-64 and US-54 and the two days remaining in your trip to head for home.
Even this is overloaded, and I don't think you could do it all, but it gives you an idea of what's available and within reach of a four day loop.
AZBuck