[lunarhighway]

vortex generators when applied well might do something, but it will depend on the car, on certain sedans wing spoilers might work, but again, you'd need a wind tunnel or at least on road testing to be sure, and chances are it hurts rather than helps.

flatblade wipers for older cars are quite nice, they won't have a huge effect, measurable effect, but 'im quite happy with mine, they do wipe well and last quite long so next time you need new wipers you might look for them.

an interior antenna that sticks to the front window is another nice thing.
again, an exterior antenna is usually the last thing to go when tweaking aero, but an interior antenna has 0 aero drag, so form that perspective it's very effective. got mine as my prev car didn't have an antenna and i didn't want to drill the bodywork to screw up the aero.

for some high performance cars custom undertrays are available, although most of these cars originally come with a plastic equivalent.

diffusers are also available for some cars. but again it might be very impractical to install.
moon disc hubcaps also come to mind, although some 80's - 90's opel/vauxhall hubcaps where very smooth, with only some small cooling holes near the outer edge, you might get these for free from a scraped astra or whatever.

custom mirrors might also work but again, you can't be sure, exept if they're really smaller than the originals, but likely they won't be

lowering the car a bit seems to reduce drag, so i'd nominate lower springs as a functional "aero" device.

i doubt however there's a lot of bodykits developed with drag reduction on mind, the best of these will incorporate functional spoilers and ducts that aid engine and brake cooling and generate some downforce... none of this will help your FE though, and mostely will increase drag.

i've seen a few (very few) subtley tuned cars wich had custom bumpers and sideskirts that where very smooth and blended well with the stock car... no extra wide body kits and wings that made the car look like it was about to transfom into a japanese robot, just something that seemed smoother than the stock item.

again, no huge gains here i think but they would help a little depending on how well the original bumper was made.

so i don't think there's much out there that will really return the money you'll invest in it, but some things could be considdered if the stock item needs replaceing.