Niagara Falls, in New York.
I've got to get there some day. Maybe this summer. I was born in Westchester NY, so it's far from where I've lived, but I've really got to go see it. Amtrak is the train line. Goes from NYC up the Hudson River, gorgeous in the Autumn, loads of trees changing colors.
Probably Grand Canyon too, is what the western people would say.
If you're looking for pre-Columbus, and are in New York State,
https://www.newyorkupstate.com/attractions/2015/09/5placestoseemastodonremainsinupstatenew_york.html
There's been a lot of Wooly Mammoth fossil finds, if you like dinosaurs.
The reason there aren't a whole lot of historic buildings is not because there's so much money to tear them down and build new ones, but because our weather here in the US can be brutal, blizzards, hurricanes, tornadoes, loads of stuff that doesn't keep buildings standing forever. Please watch your timing of your travels, avoid hurricane and tornado seasons (presuming you're not traveling during blizzard season :) in southern and midwestern places, please.
My Dad and stepmom lived in Australia (were originally from northern Westchester County too), and when they traveled back here to go sightseeing, they found there was a nationwide national parks pass they could get for cheap. Dad drove the camper van all over the place, the parks sure lost money on those two, that summer :)
Learning about Ben Franklin ahead of your Philadelphia trip will make it much more meaningful. He was our best character, mostly :)
I have an uncle who likes to plot out trips to go see historical theatres. Pinpointing a specific interest can help, since the country is big.
If you're traveling through Hartford, Connecticut, the Mark Twain house is great, I hear.
My personal favorites are Yale University in New Haven, Connecticut (stonework of buildings amazing, my grandpa went there), the United Nations (I'm an online volunteer, usually very good energy for NYC and tax free shopping downstairs since it's international land and not NY-taxable, plus it has a Delegrates Dining Room that the public can eat in, for $35 during weekdays you can get this amazing international buffet and river views with the delegates, we used it as a meeting place for my stepbrothers in from Australia and MExico, their families, and my Dad and stepmom), and Hayden PLanetarium in New York City (IM' an astronomer), and most of the Hudson River views from different parks like Bear Mountain in NYS. I've been through Providence, Rhode Island and it looked like a European city to me, very different than most other places I've seen in the Northeast. I was just attending my cousin's wedding so I didn't get to see much of it, but that might very well be a hidden gem.
Boston has a lot of history, another train trip, no driving cars there nor in NYC. No fun.
If you're traveling where there's wide open spaces, you may want to search for "star parties" where astronomy clubs put on big viewing events, could be awesome way to meet people.
If you like cultural stuff, there's a lot of specifics you can find. Example is if you like Irish Dance, there's a whole web site that tells of where every Feis (pronounced fesh) is, nationwide. Lots of specific cultural festivals, lots of Ockoberfests in the right places at the right times :) Whatever you find to be fun.
I have found that the Google Fi phones that switch networks as you travel are the best for us now that we're in a rural area of New Hampshire. Different mobile phone networks dominate in different towns or areas of the country, and you can lose your GPS/Sat Nav signal if your phone can't change networks, sometimes. Also really good price and supposed to work internationally.
Have fun, and have a great trip!
-Alison