Yes, you can—and are encouraged to—take a backpack as a carry-on when boarding a Carnival Cruise. Upon arrival at the terminal, your large suitcases are typically handed over to porters to be delivered to your stateroom later in the afternoon. A backpack is essential for carrying "embarkation day" necessities such as your passports, boarding passes, medications, and a change of clothes or swimwear. This allows you to start enjoying the pools or Lido Deck buffet immediately, even if your main luggage hasn't arrived at your cabin yet. The only restriction is that your backpack must fit through the security X-ray scanner, which is similar to airport security (standard dimensions are usually around 24"W x 16"H). Note that while you can carry on your own soda or wine (within specific limits), these must be in your carry-on bag and not in your checked luggage for inspection.