In Orlando, casual, comfortable clothing is best. Men need a jacket and tie in only a handful of restaurants. Restaurants with dress requirements are strict about enforcing them. If you plan to eat at a nicer place, check on dress codes so you aren't caught unprepared.
It can get quite cool in December, January, and February. In general, though, be prepared for a range of temperatures in winter by packing clothing that you can layer.
In summer you'll want a sun hat and a rain poncho in case of sudden thunderstorms (or for watery theme-park rides). On hot summer days the perfect theme-park outfit begins with generous amounts of sunscreen, followed by shorts made of a breathable, quick-drying material, topped by a T-shirt or tank top. At any time of the year, pockets are useful for FastPass+ tickets, cell phones, and park maps.
Give everyone his or her own bag. Individual suitcases (preferably different colors—red for one, blue for another, etc.—and preferably carry-on size, so you don't have to check bags and pay extra fees) help you find items in a hurry. This will make life easy at the airport, as even little kids are capable of toting around a mini Pullman—they often enjoy it.
Pack one communal carry-on. If logistics forces you to check luggage—despite hefty bag fees—packing a family bag of essentials saves heartache if some of your luggage is temporarily lost. Include an outfit for everyone, as well as prescriptions and other must-haves. A communal bag also simplifies car travel, giving quick access to overnight necessities for midway stops instead of having to unpack the whole car.
Consider creative carry-ons. Soft coolers that don't go beyond airline size limits make great carry-on luggage. Roomy and crushable, they're perfect for nonbreakable items (such as those extra outfits) and help you cut your food bill by becoming picnic baskets for snacks.
For Car Travel
Check the trunk. Trunks are prone to dampness—wetness from one big storm can remain in your trunk for long periods and then creep into luggage. Line the bottom of your trunk with a waterproof tarp or, better yet, with some kind of slightly elevated platform that allows air to circulate between the base of your trunk and the goods stored in it.
Separate your "getting there" and "there" clothes. If your Orlando trip involves a hotel stay or stays along the way, keep the clothing you plan to use once you arrive in one piece of luggage at the back of the trunk or under other bags. Put the clothing you need during the drive in lighter and smaller duffel bags or luggage stowed in an easy-to-reach place.
Keep essential road-trip items handy. Pack the emergency kit last and in a place where it's easily accessible. Also keep books, electronic entertainment gadgetry, and the cell-phone charger handy.
Organize with plastic drawers. You can buy cheap, stackable plastic drawers at most department stores. Put a few of these in your trunk, filling one with books and maps and another with toiletries, and you'll have an efficient system for even the longest trip from home.