Yellowstone, we did it last year as a family... kids 4 and 7. Tons of wildlife to see. Buffallo everywhere, even in the road with INSANE people gong near them. Sulfer, exploding holes in the ground
everywhere...rotten eggs, as my kids would call it. We even saw a pack of wolves, elk galore and beware, an overturned trailer from not watching the road, sharp drop offs along the sides of narrow road. Tons of traffic. We stayed in west yellowstone and hoteled it. (no trailer then)
Sorry to admit it but I can only take so much scenery and 2 days in the park was enough for me. A must is CODY, WY to the east, Buffalo Bill Historical Museum. The alternate route from Cody to Sheridan on highway 14 (?) is unreal. STRAIGHT UP and curvy but unbelievable. Elk and Moose and no one around. Then if you are heading there in July, a must is the Sheridan WYO rodeo. Awesome small town rodeo with National Champions... and I am not a cowgirl.
In the other direction, hit the Lewis and Clark Caverns. Mighty hike up a hill to get to them in the heat but well, well worth it! Bozeman, MT is also a treasure with a great Dinosaur Museum, the Museum of the Rockies.
By the way, there is a webcam of Old Faithful that shows it going off, you can get online (we have verizon broadband wireless) and see when the next shoot will happen without getting there and waiting 90 minutes for the time to come. (Can you tell I have ADHD?)