If you would like a relaxing night after a busy day of sightseeing, check out what might be going on at the John F. Kennedy Center for the Performing Arts.
Other options in DC are limitless but you may enjoy the U.S. National Arboretum, a tour of the Bureau of Engraving and Printing, Ford's Theater, the Supreme Court, and the National Aquarium.
The best piece of advice that can be given is to plan out your trip in advance so that you will know which places you would most like to visit and even which restaurants you may enjoy. It eliminates a lot of wasted time looking for something to do or finding food at mealtime when there are almost overwhelming possibilities available.
Speaking of food
Naturally in a city of this size and one so culturally rich, you will be finding innumerable selections for dining. There are just as many diners and mom and pop establishments as there are very extravagant and expensive upscale restaurants. Many of the eating places in the city are award winning and the range of cuisines that can be found is remarkable.