[Christ]

This is still "heavy over shirt" weather here.. It's been hitting 30's pretty regularly, now.

OP - It's winter... grille block. Aside from the mods that most people consider to be "hideous", it's probably the single best vehicle mod you'll be able to do.

On top of that, air dam, wheel skirts, smooth wheels, thinner tires (which are actually better in snow anyway). It's absolutely not economical to replace things that do not need to be replaced. Since you're in it solely for the fuel economy, you're in dire straits for mods that will be cheap enough to pay themselves off, I assume. In that case, and with your short "window of opportunity" for modding (next spring/summer, right?), you're basically looking at free mods.

If this isn't the case for you, and you don't really care about spending some money, I apologize for assuming.

So, to categorize:
Grille block, full/partial, start small, and test it out. Do what's comfortable.
Wheel skirts. If you're comfortable with the look, do it.
Smooth wheels. You can easily make some caps for them, or buy some pizza pans if you have steelies. Added benefit: if you're down a pizza pan, steal it off your SUV.

You can find foam or something to fill/stuff the gaps in the front end, as well.

Engine Blanket - ask Frank Lee, he's got 'em, and his were "free" (recycled).
Block heater.
Sidewall max tire pressure - Contrary to popular belief, harder tires are better in the snow than underinflated tires. You want a tire that will cut through the snow, not pack it down underfoot.
Thinner tires will provide lower Rolling resistance, and will help with the above point.
Remove the engine's fan, and get an electric one from the Junkyard. You won't find a direct fit electric fan, but you can make just about any one work if you can get creative with zip ties.

Power steering, remove/etc.

Alternator switching - Let it run while you're idling, but not while you're on the throttle. It's a simple circuit, controlled by a microswitch behind the accelerator pedal, and a normally open relay controlled by the microswitch, which cuts the circuit to the field wire.

Replace your engine oil, and front and rear gear lubes with synthetics. Replace your transmission fluid with Dex-VI, the newest (synthetic) fluid available for GM transmissions, applicable for use where any Dexron is specified. Use the lightest oil your engine is happy with. In dead winter conditions, you're probably OK with 0w30 or 0w20 Mobil1, which both consistently outperform most 5w oils.

On top of all this, of course, is driving habits. Change those and you'll see the best gains. I have next to no mods on Cara, and when I get a normal tank in, I get between 27-29MPG just by adjusting my driving. I'm not trying hard, I don't even do alot of the things that some of these guys on here do, I just drive calmly, responsibly, and "drive the road ahead" as often as possible.