We have enjoyed Florida, stayed at a villa in Kissimmee with a pool, and went into the various parks and also up to Kennedy Space Centre which was a highlight. Boys were probably similar age to your girls.
Since then we've done West Coast, 3 wks, with car, Vegas, Zion National Park, Grand Canyon, Yosemite, Death Valley, San Fran, Monterey, drive down beautiful highway road to LA, home from there.
Our last trip, they are now late teens was New York, train to Boston and a rest on Cape Cod.
Loved all three trips. The first was more "holiday-ish", in one place, pool, although it was busy at the parks each day. There is a brilliant book that will help you plan that we used called:
Brit Guide to Orlando 2016 by Simon and Susan Veness
The other two trips I researched myself and then went to Trailfinders to put it all together. I can definitely recommend them, it was good to have flights, car hire, hotels etc all in one place. They have great knowledge and can generally find 3 nights for the price of 2 hotel offers.
Our West Coast was a real adventure, never stayed more than 2 nights in one place, so saw many amazing places, our favourite sight was the Grand Canyon, it's just incredible. Great family memories of us at Zion hiking the rapids
utah.com/zion-national-park/the-narrows
And also hiring bikes and cycling over the Golden Gate Bridge in San Francisco one Sunday morning.
There are some great threads on mumsnet in the travel area, in particular regarding the West Coast section, also great recommendations and ideas for New York.
I would perhaps do Florida first and then the other places when they are a bit older.