I want to go on holidays.

After a big  long day, I'm thinking about holidays(/vacation) and I wonder what is the best place to visit in United States after New York City is ? I've heard about so many towns : San Francisco, Chicago, Philadelphia, Boston, and I don't know where to go !
Best,
Thierry

Depends on your interests of course and when you plan on traveling.  The South- Southwest and most of the East Coast are really hot in the summer.   If you want to see cities those that you named are the most famous.  How about Canada's Toronto or Quebec?
If you go to New York - Philadelphia Baltimore and Washington DC are pretty close without too much extra travel expense.   I've also heard good things about Phoenix AZ and Miami FL as vacation desinations.

I want to go on holiday.
(U.S. English:  I want to go on vacation.)

After a big day, I'm thinking about holidays (vacations, trips, travel) and I wonder what is the best place is to visit in the United States* after New York City?  I've heard about so many towns: San Francisco, Chicago, Philadelphia, Boston, and I don't know where to go!

Best,
Thierry

* Also correct:  
I wonder what the best place to visit in the United States is, after New York City.

P.-S. Ne pas oublier de Seattle et Portland, mais bien sûr, j'ai un parti pris pour l'Ouest américain. :)