I just finished a tour of southern Wisconsin last week. Some pointers/suggestions that I have:
- Milwaukee has lots of bike lanes, trails, etc. so getting around and through won't be a problem. I recommend riding along the shoreline if you get a chance.
- While using roads close to the lake are a great option, there is also a trail network that takes you from Milwaukee all the way to Sheboygan. The Interurban Trail is the primary one which runs the entire length of Ozaukee County.
- The ride from Sheboygan to Manitowoc is quite peaceful and Manitowoc/Two Rivers has a very nice trail along the water.
- After that you don't have a lot of options if you want to stay as far east as possible. Hwy 42 is about it. It's a busy road, though not as busy as others in the area, but it's quite workable.
- Once you get to Door County you have lots and lots of options. It is a major tourist destination for Chicago and almost all of the services will be along the water. (That doesn't mean that you ever far from any since the peninsula is not that wide.) Note that Labor Day weekend will probably be very crowded (with correspondingly heavy traffic) throughout the peninsula since it is the last holiday weekend of the season.
- There are plenty of B&B type places along with small motels. However, keep in mind that the later end of your time frame is encroaching on the "color season" and many places are probably already booked for that time. That said, you will have much better chances of finding a place mid-week. As far as camping goes, there are two main state parks on the peninsula: Peninsula and Potawatomi. There are a few private campgrounds but I'm not sure where they are. I would double-check, but I believe Wisconsin has a "guaranteed accommodation" policy for people arriving by bike/foot/canoe.
- For things to do, the Door County Bakery outside Sister Bay is worthy of a side trip. Some of the wineries in the area have free tastings. You can take a ferry from Gills Rock to Washington Island for a nice day-trip. I really enjoy pedaling around Peninsula State Park. It is Wisconsin's premier park. You might be there during apple season so you'll have no problem finding apple treats along with lots of cherry treats. (Door County is known for its cherries.) The entire summer had a very late start, so it's hard to judge when crops will be harvested. The list goes on and on...
- You really can't go much further north and keep your trip at 500 miles and experience Michigan. You'd need to take a ferry across somewhere.
I don't know much about Michigan other than it also being a great place to tour.
Hope this helps.