We are first time cruisers too, but from what I've read, buying packaged snacks on board is pricey. We will be bringing our own candy, granola bars, chips and water. From what I understand everything has to be store-bought in original packaging or you can't bring it on. We plan on bringing an entire suitcase for this stuff, and we'll repack it with souvenirs when we disembark! I'm also hoping we can get fresh fruit on board to throw in our bags for snacks during excursions.
We are also bringing our own adult beverage, but it has to go in our carry on bag when we board.