I would do a belly pan (at least from the front bumper to the front axle).
A grill block is also a good idea, an upper one helps more with aerodynamics than the lower one. A lower one will reduce the snow packed into the front. If you make it the same color as the OEM grill it will by less noticeable. YOu could also make it on the inside side of the grill. This will reduce the aero benefits somewhat but will be stealthy.
Wiki links for mods:
Car Modifications Main - EcoModder
Belly pan - EcoModder
Grill block partial or full - EcoModder