I have to ask you: Have you taken other road trips in this manner?

What are you going to do if you cannot find a hotel for the night once you get to a specific location - or only those which are quite expensive or might be outside your budget?

I'm asking because I can think of a couple of periods in the Chicago/Chicagoland area where you'll be looking at prices $200+ night (if there is even any availability, such as with big conventions or Lollapalooza time). And parking downtown may add another $30-60+ per night.

You also might want to tag Wisconsin in this post (can you do that now that it's been posted?) as 90 and 94 go through our wonderful state as well. That being said, Madison is wonderful in the summer, if you happen to end up here on a Friday evening, make sure to stop by the Capitol square for the Saturday farmer's market, or spend some time outside at our free zoo, on State Street, or along one of our lakes. Plus hotels aren't too expensive around here, either.

Another small town worth visiting (that I also maaay have a small bias towards, since I went to college there) is Winona, MN... the local coffee shops and beautiful bluffs in the area are completely worth it. Plus nearly all the hotels in the area are under $100 a night - I suggest the Riverport Inn. Stop at Acoustic Cafe or Blooming Grounds for breakfast or lunch, and Bloedow's bakery if you get there before they're all sold out.

There's obviously pros/cons to both winging it and reserving. When we did our last trip, we had reservations and will do all future trips this way. While it took a lot of planning ahead of time, it made the trip so much easier. After researching reviews, we knew where we were spending the night, the price, the location, and what to expect (no yucky surprises). If you haven't yet booked for YNP, I would do it now as accommodations fill up fast; even at this point, you might have to stay in one of the gateway towns (which personally I find OK). The Black Hills also books up fast. And keep in mind, the oil boom in Williston, ND; rooms are hard to come by within about 3 hours of Williston, so you might want to plan your trip accordingly.

In the MT portion of your return trip you could consider a couple of detours. I-94 along the Missouri River is not terribly scenic, at least when I lived in Terry, MT it wasn't considered a scenic drive. Pompey's Pillar is just off 94 near Billings.
If you havetime time, turn south on i-90 to the Little Big Horn Battfield. Then, when you reach I=15 in Butte, go north and make a visit to Great Falls befroe takin US 89 to Glacier Nat. Park and cross either on US 2 or Going to the Sun Road. You can then stay on US 2 until it joins I-90 in Spokane. Or, take US 95 off the 2 at Sandpoint, ID and rejoin 90 in Coeur d' Alene.

Funny that the Going to the Sun Road would be mentioned.
Yesterday (6/1), the GTTS was open for 13 miles from the park gate at St. Mary. I saw a picture taken on Friday with snow banks on each side more than 20 feet high. The picture was taken within 2 miles of the Logan Pass.
If you do drive US 89, beware of the horses that linger on the highway near MP 20. They were busy licking salt off the road.
If you go through Browning, make a stop at the Museum of the Plains Indian near the intersection of US 2 and 89 ($4 PP cash)

You don't say how much time you want to spend in the Black Hills, but if you have time I'd highly recommend spending some time in Custer State Park. If you look at a map of the Black Hills you will see a loop route that will cover the park, the Iron Mountain scenic byway and the Needles Highway scenic byway, and it will take you right to Mt. Rushmore. These drives are spectacular!

We've stayed at The Roost Resort cabins before, they are about 3 miles west of the entrance of Custer State Park on Hwy. 16 and are cute and reasonably priced. In Custer, Baker's Cafe has very good breakfasts as well and wonderful pastries.

If you need a break from I-90... in Sioux Falls, Falls Park is a very pretty area with many waterfalls. If you like kitsch, the Spam Museum in Austin, MN is a fun stop (and actually a really good museum to boot). If you really like roadside kitsch, you can go thru Wisconsin Dells and pick up the interstate again just south of the town.

Hi Charlotte, I just want to echo the post about the shortage of hotel rooms in N.D. You can get rooms in Bismark/Mandan but nothing further west of there until well into Montana. We stayed in a BnB in Glendive called www.charleymontana.com . We also took I 94 one direction and I 90 on the way back. Also if you can get a room at old faithful Inn it is worth the drive from the north. Enjoy your trip, ziggypop

Answering some of your questions (after the fact, I am sorry)
@amandarawrrr They only let me tag a certian amount of states in this post, and Wisconsin was one of the ones I had to leave out.
@exiledprincess... yes we have done road trips of this type before and to save both time and money we plan days in places like Chicago, staying close enough that we can drive in the next morning. I know that Chicago has way more than a day trip can fit in, so when I have more time I will fly down there and really explore.
Thank you all for your tips and advice, you were very helpful. I am sorry I am just having time to respond now. For more details see my trip report that is coming soon

Rather than Mall of America, visit Minneapolis, a beautiful city, lots to see and do.