You had some really good tips so far!
Call the car rental company or go online (depending how you made your reservation) and see about the car size you can expect. Most on-line sites have luggage/people/car seat icons that let you decide IF the car will be big enough for you and your things. You have plenty of time to do that, I believe you can change even the day you pick the car up BUT don't count on that. Make sure you have credit card in the name of the driver/renter with you, some car rental places do not take debit/credit cards. Or call and ask ahead of time.
Make sure to have all your ID's in order. Good time now to check to make sure your license hasn't expired or will expire soon. It is not valid if it has expired (I had that happen right before a business trip and I had to rush around the day I was to leave, now I check). Wear slip on shoes, no belts (unless you can take it off and keep your pants up!). Keep you ID's handy in the airport, you will be asked for them a zillion times. Also, make sure you DO NOT bring anything that is not in a bag (hand lotion, lipstick, perfume), take a few extra quart size bags with you in your purse just in case something has to go in at the last minute. CAUTION, If something liquid is over 3 ounces you will have to throw it away! Don't buy a drink before security check points in the airport, buy a bottle of water AFTER you're thru the check point. Food is okay, drinks are not. I really try to carry on little or nothing but my purse, makes for a smoother time thru security. Of course, I don't have to travel with little ones anymore so that makes things a little easier. OK, a lot easier!
Things you can pack now are things like a first aid kit, meds that you may need (like cold and flu meds, ammodium, tylenol or advil), travel size mouthwash, toothpaste, shampoos. I always have those things available here at home so I pack them ahead of time but if you don't then you have time to gather them.
Save extra room in your luggage for things you will buy and bring home with you. Suitcases that are checked on the plane cannot weigh over 50 pounds. You will pay a fine (25-50 bucks) if your suitcase is too heavy. I think most airlines allow for 2 checked bags per adult flying, we usually take one almost empty so we have plenty of room to bring home our purchases.
Most important of all.....if you forget something, no worries! There are all kinds of stores close by WDW. You can buy whatever you forgot. It happens to us every now and then. We get somewhere and someone has not packed socks or underwear (usually the men in my life, no offense to the men out there).
Have great time! Relax and enjoy WDW, make as many memories as you can! :wave: