When I was kid in southeast Idaho, I had a very unique playground in which to spend summer vacations, and my parents and grandparents made sure that we saw it all. But I have to admit that there's a special place in my heart for one place in particular: Yellowstone National Park.
Now, I'm sure that there are scheduled tours and "people who know their way around" Yellowstone, and I'm sure that those are fantastic ways to see this ancient geological site that I have never experienced. I've been going there since before I was old enough to even know where I was. And if there's a place on this planet that makes me understand that we are all one, it's there. You'll want to plan this trip soon -- definitely have it completed before the end of September. The weather can get unpredictable.
The Grand Loop is the only way to see Yellowstone. It's 140 miles of narrow, winding roads to traverse, so plan on taking your time. I don't recommend doing it in a day, unless you plan to see only what you can see from your car. The Upper, Midway and Lower geyser basins, Mammoth Hot Springs, Tower Fall, the Grand Canyon of the Yellowstone and Lake Yellowstone are all stops along the Grand Loop, and there are several historical areas that are directly off the Loop, including Mammoth Hot Springs, North Entrance Road, Roosevelt Lodge, Lake Fish Hatchery and Old Faithful historic districts.
There are plenty of places to stay in Yellowstone, but to truly experience the park and all of its charm, I think your best bet is the Old Faithful Inn. Go in the West Entrance to the Park and head south, stay the day there while you take in the natural beauty of Old Faithful and all of its trails and geyers near there. Then on day two, you can make your way around the loop to the east to Yellowstone Lake and north toward the Grand Canyon of the Yellowstone, where you can see the Lower Falls and Upper Falls of Yellowstone, just 2 of the 300+ waterfalls in the park. If you want, stay the night at Canyon Lodge, too. After that, make your way to Mammoth, and experience historic Fort Yellowstone. There are more than 1,300 miles of trails in Yellowstone, so take your walking/hiking shoes. The geyers and hot springs are truly amazing sights (note, however, if you're sensitive to the smell of sulpher, take some lavender oil to dab under your nose).
Yellowstone was the first place I saw buffalo, a bear, a moose and her calf and dozens of other types of wildlife. You can usually tell when there's one near, because you'll be driving along, turn a corner and it will look like a 20 car pile up in the middle of the road, with tourists anxiously scrambling out of their cars to get a better view of whatever animal is grazing. If I can offer any advice, be one of the patient ones who either winds their way through the stopped cars or waits for the roads to clear. As docile as anything looks on NatGeo, remember that you're on their turf when you're in Yellowstone, and they're definitely wild animals.
There are few places on earth that can match Yellowstone. You'll have an opportunity to peer directly into the earth and get a note on how it all comes together. Take nothing by photos, leave nothing but footprints.