Our first cruise after having our son, I packed a lot of extras. I found I didn't need/use most of them. I had a ton of snacks (pouches mainly), but I learned I could grab extra bananas and boxes of cereal at breakfast. I had toys like crazy on the first one, but he was too wiped out each evening to play. I brought a couple of cars and an iPad loaded with movies/tv shows the next one.
Non-negotiable items: some form of transportation for him (either a stroller or backpack carrier), sunscreen, diapers/wipes, those scented diaper bags to keep the room from getting skunky, anything required for sleep (i.e. Pacifiers, sleepsacks, portable noise machine, etc), sample size dish soap for sippy cup (only bring 2), disposable bibs and not much else. I always pack a makeup bag full of medicine for everyone - Tylenol, pepto (for adults), allergy meds, Benadryl, neospirin, band aids etc.
My biggest advice is to pack consumable (diapers, wipes etc) in a box. You won't be bringing them home, so you don't need a suitcase for them. It saves room to unpack them then throw out the box on the first day.