re: Ideas for a trip with older teens for 3-4 full days.

San Diego used to be my go to recommendation until we went to Boston/Cape Cod last year. Really enjoyed that area.

"The upper Michigan peninsula has always interested me, but not sure it would have enough memorable spots for the teens to make it worth it."

We did a foliage road trip of northern Michigan in October this year and I think you could fill 3 or 4 days there. Google Michigan Hwy 22, Sleeping Bear National Lakeshore, Mackinac Island, Tahquamenon Falls State Park, and Picture Rocks National Lakeshore. Mac Isl doesn't allow moterized vehicles; you bike, walk or take a horse drawn carriage/taxi. You can fly there but most take the ferry out of Mackinaw City or St. Ignace. We did a day trip but you can stay overnight but you would probably need to book soon.

Also, on the NP there is Whitefish Point and the Great Lakes Shipwreck Museum is there. Whitefish Bay is where the Edmund Fitzgerald was going when it sunk.

We used the GuideAlong app and purchased the Michigan Lakeshore M-22 tour. As you drive along the narrator describes upcoming places to stop. You don't need internet and that tour cost $16.