I love Kennywood. Have you seen Adventureland? It was filmed there. I went in September, so it wasn't busy. Not sure about July.
I live in Erie, Waldameer is a great family park. Conneaut Lake Park is sad, because it's on it's way out. I went last year, and a few rides were open.
Of course, there is always the option of going to Kennywood, then the next day stopping in at Conneaut Lake in the morning, then heading up to Waldameer (I assume it's a road trip).