My first impression; yeah right, like some vacation person would know about what might influence kids in a long term way. Then I looked at the list and what I saw as a very young child.
Lincoln Memorial: Lincoln is my favorite historical figure to this day. It is the only thing I remember - except from getting to my relatives house 0 on our trip to DC when I was little.
Grand Canyon: At 10 years old I can 'still' remember my first impression upon seeing it. It mesmorized me. I was fearless there as well - lots of pictures of sitting on the railing with a long drop.
Redwood: I have never ending (vague) memories of this as a four year old. What the heck? Sequioa at age nine still resonates with me.
Niagra Falls: I cannot forget our trips there, but in my young kid mind I blocked it out a bit (the horseschool falls) because I had a hard time accepting that the Cannadian falls kicked butt on the American falls. Of course my dad was a hard core Marine at the time. FWIW, I know a guy who did the "barrel thing". He would never do it again and is still paying for it.
Independence Hall: I was nine. I remember the bell, and the Declaration of Independence - like it was yesterday.
Freedom Trail: We took the long version. The little feet on the trail were cool. My feet hurt a lot! I "got" most of it and am a fan of history to this day.
Disney World - Replace with Disneyland for us old west coast folks and you have the same thing. I remember my first trip (fragmented) and I was four.