I'm assuming you are coming from Florida. "Spring" in Wisconsin is not like it is in Florida... we can have blizzards as late as March and April, and what we consider minor snowfall as late as June. The UP is much worse. Depending upon your route, you might encounter some river flooding in spring, but mostly that is in the Western part of the state on the Wisconsin and Mississippi river systems. You might see some of that in Portage. These things can cause detours.
There are three main routes into Green Bay (the most logical wayfarer's route to the UP), they are highways I 41/US 41, I 43 and WI 57/WI 32. I 43 us the newest highway and usually has the least amount of traffic from Milwaukee... mostly it was built to handle football traffic when the Packers have a home game. Try to avoid traveling on those days if possible. An old army buddy of mine used to sit at the drill hall for three hours to wait out after-game-day traffic. If she left early, it was not unusual for her to take anywhere from two to three hours to dive to her home in Appleton sixtyish miles to the South. Here is a good link regarding the highways around that area:
Green Bay - AARoads
If you stay in Green Bay, avoid the Tundra Lodge... they have a local reputation for bed bugs, but if you like a really nice water park including a "lazy river", hot tub and a pretty good water slide, it might be worth the risk.
I would suggest avoiding Door County... it is mostly a tourist trap area with exorbitant prices on pretty much everything, though there are a few things there to look at. You can pick your own cherries in late spring if you want to, but then you gotta figure out how to get them home again without spoiling. I'm pretty well set up for canning if you wanna stop by and spend a day or so in the kitchen. Got a black powder range in the back yard. In between canning loads, you can shoot from the back porch.
In Green Bay there is Herritage Hill State Park, which is definitely worth a visit.
Heritage Hill State Park | Home They do have re-enactments, but you gotta check their schedule. There are early buildings including those originally within Fort Howard, an operating letterpress shop, an operating blacksmith shop, a fur-trader's cabin, an early law office and various other buildings I cannot recall.
There is also the National Railroad museum. They have Eisenhower's train from WWII, a Union Pacific Big Boy locomotive that was fully functional, though not in operation last I heard, plus lots of other items of interest.
National Railroad Museum - Green Bay, WI 54304
Take US 41 into the UP, in my opinion, because you might want to the see the Peshtigo Fire Museum
Peshtigo Fire Museum – Official website of the Peshtigo Historical Society.
If you choose instead to take WI 32 out of Green Bay, then you will want to see Waubeno, WI, where they have, among other things an actual working steam log puller from the early lumbering days. Further North you will come to Laona, WI which has an operational steam train that you can ride in... you will experience a "train robbery" while in transit to the historical area if memory serves and last time I was there I trained a young blacksmith who worked in their smithy.
Historic Galloway House and Village
Rhinelander is not a bad place to stop... you will learn all about the "hodag"... and there have also been bigfoot sightings in the area.
I have only been to the UP on one occasion, an unpleasant trip to pick up my daughter from the Escanaba Police Station in a 20-year-old car with bad tires in a snow storm. Escanaba in the Moonlight is only worth seeing in your mind's eye while listening to the song of the same name.
Traveling to Michigan can be an interesting experience. The laws for the UP are pretty much made in Detroit where the "trolls" live (they are "under the bridge")... the locals will tell you all about that. Having a handgun in Michigan is generally illegal, even when it isn't. On the few occasions I visit Michigan, I bring a muzzleloader with me in the trunk of the car in a case. Unlikely for me to encounter bigfoot or some other hazard that could require a firearm, but I wanna at least have SOMETHING to deal with threats should we have a nuclear war while I am up there separated from my own infrastructure & contacts.
Good Luck!