Kyle says : "Have you noticed that everyone at Cedar Point is from Michigan"
This is because Cedar Point is the Closest Major Amusement park for most people from the Detroit Area. (I used to live in Redford, just west of Detroit. 
) The next Major Parks are (in order of distance) Michigan's Adventure and Geauga Lake. I have done all of these parks as day Trips when I lived there. I also did weekend trips to Kings Island, Indiana Beach, Kiddyland, Great America, Wyandot Lake, Camden, Kennywood, Conneaut Lake, Waldameer, Canada's Wonderland and Martin's Fantasy Island, as well as the Now Defunct Idora, 
Americana and Fantasy Farm Parks.