I agree with the above post, why not check out something uniquely "US" while you're there. I love the desert states of the southwest. I did an awesome hike in Zion that I'll never forget (it was called Angel's Landing), and I loved Monument Valley and Mesa Verde. Not sure you'll be able to add them to your trip this time, but I definitely urge you to keep them in mind for a future US trip
Well I'd certainly add New Orleans to the list for nightlife, food and a totally different view of culture in the Americas.
Also I'd consider getting to some more uniquely US Places like Charleston or Savannah both of which have a different charm.
I also wouldn't try to cram too much in. There just is way too much to see to get all of it in 21 days.. pick 2-3 places
That's a very good point. New Orleans and Charleston both have a unique culture and even language that is not like anywhere else in the US. Or anywhere.