There are so many good suggestions already in this thread. Electric fan and synthetic fluids come immediately to mind. They're low cost (relatively) and almost certain to help your FE. In any case, use the thinnest fluids that GM recommends for your climate. Maybe thinner, if you have good reason to feel confident that no damage will result.
A standard trans conversion would likely net a large gain but you have to gauge that vs. the cost and complexity of the project.
i_am_socket mentioned a boattail.
I'd estimate that on a vehicle of that size and shape a boattail would be a big aero improvement. Of course, it would be big, period. Definitely worth considering. Check out basjos' Civic for inspiration.
basjoos civic
Check your state's motor vehicle regulations first. Here in CT I was told you can't install anything permanent rear of the rear bumper. I wasn't going to get into an argument about tow hitches; I just wanted to hear what the guy would say. Check your state before cutting/building.