Chunky knits (or grandma sweaters) are perfect for Fall. They can be worn in so many different ways and are easy to layer with.
Pairing a chunky sweater with delicate shorts gives the look an off balanced feel in all the right ways.
Depending on how warm of chilly it is... you could always add tights or leggings with a mini skirt.
It can look preppy at first, but try a button down top underneath a sweater. You can make it more your style by adding black leather pants (rocker) or even a maxi skirt (boho)