Lower Layer: So long as it's thin, most any top works: T-shirts, tank tops, sweaters and button-downs. Something too thick, though, like a chunky knit or scuba sweatshirt, will bulge beneath the jacket's trim cut.
Top Layer Option: To transition this piece from Indian summer to late fall, try a slightly shorter hip-grazing vest. For a more casual feel, pick something puffy and sports-inspired; sleeker quilted styles are better for commuting to work.
Styling Tip: Because most blazers have narrow sleeves, don't try to roll them with something wrist-length beneath—it looks sloppy. Instead, peel the cuffs back a few inches to let the contrasting fabric peek through.