A good rule of thumb for dressing when visiting West Point is to dress conservatively and a notch or two above what you would wear in a casual civilian setting. (Example for summer: going to a church picnic.) It's going to be hot and a long day, so dress with that in mind. I'd suggest:
Ladies: Dress shorts, culottes, or summer skirt - nice summer top.
Men: Nicer shorts, khakis, polo shirts, etc.
Try to stay away from anything too flashy or revealing. Definitely wear something comfortable - especially comfortable walking shoes (sneakers are fine.)