Although I prefer the classic/modern style in the office, it happens that I dress myself casually sometimes. Of course, provided that I have no business appointments. Because here the casual style is out of place. Finally you want to look trustworthy and competent.
In my current example I wear flat loafers with jeans and a simple white blouse. Moreover a black blazer with sleeves rolled up and you have a cool look, which works in the office and also in the spare time.