If you're landing at 5 PM then having a car just means you get stuck in rush hour traffic. The Metro is by far the easiest way to do what you need to do; it has a stop at DCA and can take you straight to the Mall. By the time you get there it will be close to 6 PM, so depending on when you're traveling (i.e. how much light you will have), you may want to plan out in advance what you want to see. There could be a fair amount of walking involved; you may want to consider how much luggage will you be hauling around with you, and go to your hotel first to dump it.