My Husband wore a suit when we went. He looked perfect, not over dressed, but not too casual. I wore a black linen dress with a light sweater, and I felt a little under dressed, most women were in cocktail or evening dresses. This year when we go my husband will wear the suit (poor guy, it will be hot in August) and I am going to wear a slack cocktail dress (very simple, but a little dressy).
I think a tux would be a bit much. An evening gown could work, but I would try to dress it down a little.