In AZ, I'd probably avoid a grille block, but you can almost certainly improve the underbody to some degree. Here's my belly pan:
Smooth wheel covers are one that you might like and often draw positive attention.
You might do external mirror deletes, and get some small mirrors to put inside. This cuts down on both drag coefficient AND reduces frontal area.
Wheel skirts on the rear would be helpful.
If you're feeling particularly adventurous, a partial kamm back or even boat tail would likely show large improvements. These are highly visible and often rather sketchy looking modifications though.