Get the carpet powder that is scented and you vacuum up. you may not like strong scents for your living room, but it's great in the car! it lasts for a while too.
I also use Febreeze to freshen up my car every once in awhile. However, some folks might like this:
Take a handful of whole coffee beans, wrap in paper and keep under your seat. When you park outside, they will warm up and fill the car with the fresh scent of coffee.
A few drops of your favourite essential oil (Recommend Rosemary to keep you alert whilst driving) on dampened cotton wool ball and put it in the ash tray part of your dash. You will have to refresh it every few days but its a pleasant and natural way to freshen your car. Peppermint and Orange is good too
When you know the car won't be driven for at least 12 to 24 hours, place several small bowls of vinegar in the car at various points. Carefully remove the bowls and dump the vinegar after 12 to 24 hours. The vinegar smell will disappear shortly, along with any other nasty smells your car had.