Do you have appropriate clothes and shoes? You will be walking a lot; good footware (not just "cool") can make the difference in a good trip and a nightmare. Business dress clothing will be required when attending the United Nations. ( See Delegate Information)
Spending Money (even though food and lodging are provided in the cost of the trip, there are always incidental expenses and the "absolutely essential" souviners.
Any required Medication.
Pack light - dirty clothes take up more space than clean ones and you will be bringing back more than you take.