One of the best trips we did was a 7 day road trip across South Dakota. It might not be as doable from Michigan, but there is so much to do! We spent a day in Sioux Falls, drove up to DeSmet were Laura Ingalls Wilder lived as a girl, down to Mitchell to see the Corn Palace, over to Chamberlain on the Missouri River where there is a great Native American Museum, a stop at Wall Drug, and then several days in Rapid City and the Black Hills. We didn't even come close to seeing and doing everything there is to do.
I grew up a 100 miles from the east entrance into Yellowstone Park and I've been there too many times to count (probably at least once a year when I was growing up). Going back with my kids and my niece and nephew a couple of years ago was so much fun! Don't plan on the entire week in Yellowstone. Also take in Teton National Park, Jackson, West Yellowstone in Montana, and Jackson Hole. Plan ahead!!! Even at this point you probably won't be able to find lodging in Yellowstone. I would plan to stay in Cody, WY one night and drive in through the east entrance. Drive through the southern half of the Park and spend a couple of days in the Jackson area. You will drive through Teton Park on the way there and it is pretty accessible from Jackson. Then do the northern part of the loop and spend a night or two in West Yellowstone, MT. You can finish the loop back to Cody. There is an awesome museum there and a night rodeo. It is so beautiful! If you have the time (a good solid 2 weeks) do a road trip across South Dakota, over the Big Horn Mountains, and over to Yellowstone. You will drive through God's country.
We have also loved San Diego and Coronado Island. We've been there twice.
Washington DC has tons to do and a lot of it is free. We had a great time there and I want to go back again now that our kids are a little older.
Colorado (Denver, Estes Park, Colorado Springs) has a ton to do. I can't wait to go there with my kids.
We are hoping to go to San Francisco this year.
We had a lot of fun in Chicago.
We also had a lot of fun in the Louisville/Lexington, Kentucky area.
With a 9 year old who is probably studying (or will be studying) US history, Boston or Philidelphia would be fun trips.
We loved Portland, Oregon and Seattle, Washington.
There are so many places to go!!