"American culture" is not monolithic. You can't go to Miami or LA and think that's "American culture."

FWIW, "culture" in most people's minds means museums, theaters, and live music. Not malls, spendy nightclubs, fancy hotels, and quickie tours of landmarks.

Again, "too cold" is relative. How would they feel if I told them I'd never go to Japan because it's "too cold?" It might give them something to think about if you took them to Chicago or New York and they could see all the people on the streets even when it's "cold." And it's not like you are going camping, is it? If they only want to see the country in short sleeves, they would have planned a trip sometime other than December, IMHO. So give them some credit, tell them to bring warm clothes - a sweater or two, long sleeves, a warm jacket, and hat and gloves, and they'll be fine. They can buy souvenir sweatshirts or something if they want warmer clothes.

If they are planning on staying in one or two cities for the duration, New York or Chicago would be the best choices. From there you can take day trips to many places (NYC more so than Chicago). You can easily kill 3 weeks in NYC if you have half a brain. I wouldn't want to spend much more than a week in DC, but it's worth a week at the very least.

The Grand Canyon is mind-blowing if they've never seen much but Mt. Fuji. Yellowstone would be cool in winter, but I don't know that there is enough accessible features for even a week in December. But the drive through the Tetons would be amazing. Mt. Rushmore is a one or two day visit in winter if they aren't outdoors types.

Colorado would be a good trip. Take a train from Denver, stay at a ski resort, learn to ski and see the mountains.

Hawaii isn't America. It's nice, but it's a tropical Disneyland - cool and all, but hardly representative.

 

OK, so after discussing with them it's going to be either CA, DC, or NY after Vegas. They've been doing their own research, so that makes it a little easier.

They also said they want to try IHOP and Popeyes chicken. Haha...

 

notice the "excluding"