Hello!
Agree with the above, follow his/her lead on the food and drink situation. If you get there, early don't be afraid to order something if you want it. As an AAP, I usually try to get there 20 minutes early or so and I always order something as a courtesy to the establishment I'm hosting the interview in. When the candidate walks in, I introduce myself and ask them if they would like anything. Some do, some don't - those who do have always paid for themselves and I never expect them to offer to pay for me. I wouldn't order food but a drink can be helpful in keeping you comfortable, helping the interview feel more like a conversation - which is the environment USCGA expects us to convey during the interview.
As for questions, I won't give you a list but do not expect them to be questions about your resume or things you have done, Admissions already has all of that. The point of the interview is to get to know you as a person, your traits, your character, how you carry yourself, etc. Be prepared for the questions to be more introspective. It's also okay to pause after the question is asked to gather your thoughts, don't feel that you have to rush.
For my interviews, I always offer a Q&A session at the end so I would recommend you come prepared with questions about USCGA and the Coast Guard if you have any. Best of luck!