Bike bones- plastic spacers with velcro straps to hold your bikes apart.
Extra velcro straps- for holding handlebars striaght, stop wheels spinning etc.
Some pipe insulation- cut a slit lengthways and wrap the pole of your bike carrier (unless you go for a wheel clamp style) and duct tape to the pole. At some point your bike will touch or bounce against the pole, and padding means no damage.

If you can afford to get the light plug installed (unless your car is not rated to tow). Think resale in the future. Pretty useless for anyone buying your car if they cannot tow with it.