You want to look professional, fashionable, but like you'd fit in.
Go with a pencil skirt, or black skinny jeans or something along those lines. A loose fit shirt maybe. If you read fashion magazines, check out the pics of the editors and columnists in those. They're always sort of dressy casual, but would fit in a business environment; but also make sure it fits the tone of the place. If it up market and classy, or more casual and sporty or whatever.
You want to look professional but comfortable and stylish.
form fitting dress that hits mid thigh, with subtle heels and accessories hair up look confident don't wear pants or anything like such if they say dress casual wear summer dress style so it looks like you aren't going to a dinner party but your concerned about looking like you put an outfit together ahead of time that pays attention from everything head to foot and how well it goes together.