Before you try using points (or if you find you can't use your points the way you want), try priceline...one can get some really cheap car rentals there.
I would second the suggestion to just keep the points. Most airlines only make you earn points (you may not even have to fly...just earn points SOMEHOW) only every 2-3 years to keep the account active. Eventually they can build up to the point that you can use them for something of better value.