Tips for planning my first trip to the U.S?

At 21 you may have problems renting a car. Intercity travel without a car can be expensive and a pain. We do not have the kind of intercity public transport that you are used to in Scotland. See https://www.youtube.com/watch?v=-cjfTG8DbwA&t=149s

All the National Parks are amazing; they don't get on that list unless they are. But again, they can be kind of a pain to get to without a car. I've been to Denali, Acadia, and the Indiana Dunes before it got upgraded to a National Park. The closest I've heard to a bad review of one is that Yellowstone is gorgeous but can get a bit crowded. If you can get to one and you're interested you should absolutely go.

Don't try to see everything in the US in one trip. Try to limit yourself to one region or area. The weather will likely be more extreme than you are used to even in Scotland, so check what the climate is like for that time of the year before you start making plans. It's not the heat, it's the humidity, and our winters can get pretty crazy depending on where you are, FYI.

Besides the National Parks, what jumps out at you?

/r/AskAnAmerican Thread