The UP and northern half of WI are great to visit anytime. My personal favorite times of year are late May/early June and October.
In the UP, visit Munising and the Pictured Rocks National Lake Shore, and the Porcupine Mountains area. On the west side of Delta county is Fayette State Park which is essentially an abandoned mining town. Very cool place to see. It rarely get hot in the UP. 90 is an extreme there even in July/Aug.
In WI you'll want to spend at least a few days in Door County, the "thumb" of WI. Unless you have kids, I'd avoid the WI Dells. Very cheesy and commercialized (think smaller version of Pigeon Forge, TN). Fall is fantastic in the Door. Great scenery most every where you go. Don't miss Peninsula State Park and Cave Point County Park. Stay in Egg Harbor or Fish Creek and most things are within a 20 minute drive.
