Practically an inland sea between Wisconsin and Michigan, and a great place to go to the beach in the summer. It's quite cool, and you won't chap from salt water like you will in the ocean. Practically the whole Michigan shore is a beach, and lots of it is inexpensive public beaches and state parks.
However, Lake Michigan is not a place to go swimming during any season other than summer, because it will be too cold.
My family goes to Lake Michigan nearly every summer for a week, and it's one of my favorite family vacations.