Proposal:
Smooth out the area under the front bumper, then go lower in order to improve aerodynamics. If at all possible create the piece without having to drill any existing panels for anchoring. Make it out of either fiberglass, sheet metal, or plastic.
Cost:
~$25 in supplies
Couple hours of work.
Advantages: (possibly minute, to very noticeable)
Better fuel mileage.
Better high end acceleration.
Looks better.
Less likely to damage underbody components.
Disadvantages: (possibly minute, to very noticeable)
Lighter front end at higher speeds.
One more thing to damage.
Notice how under the bumper it is slotted so air can go into the radiator and other coolers, then underneath it curves, goes flat, curves again, then notice how the oil pan and suspension seem to be so low (highlighted with a redish hue).