Un COI refers to the indirect object of the sentence. It is often preceded by a preposition. For example: Il donne un cadeau à sa mère (He gives his mother a gift): un cadeau is the COD and sa mère is the COI; Je te parle (I'm talking to you): te is the COI because we say parler à (to talk to).
I would say sort of think about what the foods are and what time of day you would have them. For example you would most likely have oysters (des huîtres) in the evening. Hope this has helped a bit!
