The Emolument Clause:
No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State.
The part about receiving a title from a King, Prince... is clear enough; the question is about the receiving a "present".
Is a "present" a "thing of value" not paid for at fair value?
Is a thing of value a well defined legal concept when dealing with immaterial goods and ideas?
What is fair value? Can immaterial goods ever have a defined, measurable fair value?
Is an honor a "gift"? Can a US President receive the honor (not money) of a Nobel prize?