There's really too much to write about the area. AKA the Midwest Golden Triangle of fly fishing. I will just add. that time of year you'll be getting into Kings (Chinook) running a lot of the Lake Michigan tribs. If you want to go for them, bring a BIG rod. ABSOLUTE MIMIMUM, 8wt (the longer the better 10ft +), you may prefer to have a 10 wt if you have one. Steelhead start heating up in early Oct but you can still find the odd early one in September.
All the rivers into Lake Michigan hold promise of nice fish. To avoid the salmon run and some of the morons that go with it, just make sure you move to above the first dam on any of the rivers. A 3-5wt will do nicely. Ants seem to work best in September or the early morning Tricos if you like fishing small flies. The PM (Pere Marquette) is the only river that has no dams so the salmon can run up a long ways. Also be aware that there are special regs on the PM especially in the flies only water...one of the better places to go for salmon if you don't like fishing with Bubba Leadripper. Do yourself a favor and grab a book or three and do some reading. There's just too much great water to talk about.
Here's an old website but you'll get the picture.
http://www.trailstotrout.com/blueribbon.html