Which park? If it's a waterpark, I'd advise the backpack, you can bring your suits and then change in the restroom/changing area! If it's MK< EPCOT, MGM or AK....I'd bring whichever one will fit what you need until you go back to the hotel for checkin(tickets, $, sunscreen, water, sunglasses, medicine-if any need to be taken during the day) Also ... how long is your flight? you will want to bring something on the plane to keep daughter entertained (crayons, deck of cards, gameboy, etc.)...also...are you taking ME...if so, you will go straight to your hotel, check-in, if the room isn't available you can semi-check in....leave all your luggage with them(so you can take whatever carryon you want, and then use the smaller one to go to the parks with!)