One option is to stay close to Lake Michigan. There is a freeway to Greenbay, but you can detour on to smaller roads to the coastal towns. Door Pennisula is my favorite part of Wisconsin, all the way to Washington Island (via ferry ride) and Rock Island state park. You can continue on north to Escanaba and the UP.
On the west side, the most scenic part is in the south, where the Mississippi valley is deepest. Further north where the St Croix River joins, roads are generally further from the rivers.
The SW part of the state is rolling hills. Most of the rest is quite flat, with the exception of the Kettle Moraine string of moraines and state parks. Devils Lake is in the middle of an old mountain range, and touristy Wisconsin Dells is nearby.
As you get further north the dairy fields give way to forest and lakes. The most scenic route up the middle roughly follows the Wisconsin River.
As in most states, the state parks cluster around the most scenic areas.