Backseat Trash Can
Having a trash can in the car is really something everyone should do, and if you make it a little prettier and noticeable it will be hard for anyone to just litter thoughtlessly. If you have kids you will know how tiresome it is constantly picking up after them, and the car is one of the main battlegrounds for this. Just hang one behind the front seat and get them in the habit of throwing their own trash away.
Polished Dashboards
You can give your dashboards a little spruce which is both cheap and easy, but can go a long way cosmetically, and the solution is only as far as your kitchen cabinet in the form of olive oil. Just grab some of this natural goodness and put a splash onto a rag, and go ahead and give that dashboard a good polish. It can really give your car a sense of renewal.