> What are must sees around here?

Unless we know what YOU want to experience during your travel, nobody can give you any meaningful advice -- although I'm sure many will try. The problem is, something I might consider a "must-see" would bore you to tears -- and vice versa. So we will give you MUCH more useful advice if we knew-
1) when you'll making this trip (since you're going to North Dakota, I'm assuming it's in the summer, but I've learned that assuming can be a dangerous habit).
2) the ages of the people making this trip.
3) the interests of these people.

> I understand there [are] ... lighthouses
As someone who has made an effort to photograph lighthouses for over 25 years, I have repeatedly found that simply because a lighthouse is present, it does not mean a visit is worthwhile. Some are impossible to view from public roads, some are too far off-shore to see, some are dilapidated, some have been moved from a glorious background, and some were never that photogenic to start with. Check out this pdf
www.michigan.gov/documents/mshda/mshda_shpo_20110713_lighthousemap_358163_7.pdf
to find out which lighthouses are near to where you'll be traveling,
and then use this URL
http://www.lighthousefriends.com/pull-state.asp?state=MI
to determine which ones are worth visiting.

I think the ride up the west coast of the thumb (or whatever they call it) is a wonderful trip in its own right if you drive through the towns along the way, many of which are upscale and most of which are attractive. We did the MI 22 drive, then US 31 from Traverse City to Petoskey and really loved it. Coming from the East, we had no idea how attractive places like Elk Lake would be. Stop frequently to eat pie.

We haven't been to the UP.

From TC take US31 north, than turn off on M-119 after you pass through Petoskey. It may be the most spectacular drive on your trip. Between Harbor Springs and Cross Village, it gwinds through a tunnel of trees and a bluff overlooking Lake Michigan.

Thre are two lighthouses that are open to the public in Mackinaw City and there are also lighthouse cruises taking you by many more. The Great Lakes Lighthouse Keepers Association is based in Mackinaw.

In the Upper Peninsula, I would get away from the Lake Michigan shore for a while. and visit Whitefish point with the Great Lkaes Shipwreck Museum, Tahquamenon Falls and Pictured ROcks National Lakeshore.

If your schedule permits, I would also daytrip to Mackinac Island.