I took the whole family to Lake St. Clair for the first time a couple of years ago in May and had a blast. We stayed north of Detroit near the lake. My son and I fished and my wife and daughter did sight-seeing during the day at places like Belle Island, Henry Ford Museum, etc. They didn't go downtown Detroit except to drive to Belle Island.

Also a little farther south...Port Clinton on Lake Erie might be worth considering.

Best of luck.