Nothing fancy, though a car shaped like a barn isn't necessarily the best place to start when looking for efficiency.
Turned the cold air intake into a warm air intake. Tires @ 45ish-psi. Grille 85% blocked. Coroplast undertray behind the front bumper. Gasket under the leading edge of the hood. Lowered ~1".
Moon discs are now left on the steelies, which wear snow tires. Snows, despite the higher rolling resistance, have minimal net impact on fuel consumption, since I spend far less time spinning my wheels.
My summer tires are mounted on XXR513s (15x7 fronts, 15x8 rears). They're aerodynamically lousy, but weigh around 25% less than the steel wheels. Coasting is slighly less effective, but acceleration is more responsive, and the handling improvement from having less unsprung weight is nice.
Still refining the performance of the nut behind the wheel...
Scangauge, air dam / belly pan, better driving. Maybe lighter wheels?