Washington is as spectacular by night as it is by day. All the major buildings and monuments are lit up. The area in and around the Mall is especially beautiful.
Although D.C. is an incredibly walkable city, the distances between the popular tourist spots is greater than most visitors realize. If you want to see the night lights without killing your feet, you have a few options. First is to just walk around but only take in select few sights. For this, concentrate either around the Penn Quarter area or further south towards the monuments. However, if you want to take more in, then you have a few options. First is to hire a taxi to take you around; negotiate the fare with the driver first. The second is to go on a conducted tour, and this is my recommendation as they cover all the major sights and log a lot more miles than you can walk on foot in the same amount of time. Here are three companies that offer night tours—you can opt for bus, trolley, and even bicycle tours.
DC Tours - http://www.dctours.us/?p=1 Viator - http://www.viator.com/Washington-DD-tours/d657-ttd?pref=02&aid=g2420 Big Bus Tours - http://eng.bigbustours.com/washington/home.html
Practice night photography before you go and have a great time!