Eric,
The trout fishing should be great at that time. With any luck, you’ll hit the hex hatch…cross your fingers. It usually starts at the end of June and works its way up river so you will likely be able to catch it somewhere at that time. I had some great hex fishing on the Manistee River last June/July. There will also be good daytime hatches (caddis, drakes, bwos, etc.), and the streamer fishing is almost always good in that area.
I’ve always gotten great reports and service from Gate’s Lodge, which is right on the Au Sable just east of Grayling. Check them out for more info closer to your trip.
http://www.gateslodge.com/
Feel free to PM me if you want more info or have any other questions.
Tim