The perfect item to take you from summer to fall is in my mind the little black vest. A little bit of menswear tailoring mixed in with a casual ease, this BDG Salford Vest ($48 at Urban Outfitters) is an affordable addition to your wardrobe. You'll easily be able to style up your t-shirt and jeans in an instant! Looking at the comments, if you like your vest to hang a little looser, then get a size up.
In addition to black, the vest also comes in a grey/brown stripe and brown plaid. Love it!