Other than what everyone else has added,
check here:
65+ Vehicle modifications for better fuel economy - EcoModder.com
Some easy ones coming to mind are;
roof rack/antenna/spoiler delete
side mirror delete (check legality, use inside mirrors/video)
smoother wheels
tire spats
rear wheel skirts
adjust ride height when changing springs
power steering delete
start using synthetic fluids on next oil change, change transmission fluid to synthetic