If you haven't gone here and you're visiting DC, definitely go. The sheer number of all of the great American art is reason enough and the building itself is great.
I have some great memories going here as a kid with my dad. There are so many fascinating aircraft that you can see up close and I took a lot of pictures of them on my Fischer Price camera. The progression of technology in air and space travel is amazing to see here, no matter how old you are.
I used to go to DC every summer as a kid to visit my uncle with my family. Even though I walked through the Mall countless times, it never felt anything less than awe-inspiring. The older I got, the more I learned about all of the history that the monuments represented and all of the history that had taken place right where I was walking. Plus, when I was really little, it just looked like the most giant space there ever was.
While Times Square is about as stereotypical as you can get for a tourist spot in New York, what makes it worth at least one visit is the atmosphere. When you're there, you really get the feeling that you're in a metropolis that's unmatched anywhere else in the U.S.