My family and I went on the Monumental Trivia Tour, round the monuments on the National Mall starting at 6:30PM. This tour really enhanced the experience of visiting the memorials as Rob provided a lot of information with regards to the events being memorialized as well as background behind the design and location of the memorials. This really helped us to appreciate the memorials more than if we had just been walking around on our own. Rob was also incredibly knowledgeable about DC / presidential history and we found the tour very interesting - the three hours flew by!!
I would reiterate advice from another review - if you visit in the summer, take bug spray - walking around the tidal basin in summer means lots of mosquitos!!