Granted, my kid was about 4 for his first trip to WDW. I saved a little bit of hassle by not taking a booster seat for the plane (I just accepted that the probabilities of anything happening was low, but if something did happen, the probability of a booster seat mattering was pretty low too). Then, when I got to FL and got our rental car, I took a chance and immediately drove a few miles in the rental car and bought a $15 booster seat at the local KMart/Walmart. Then, when the trip was over, I just left it in the rental car. Not sure it was the "right thing to do" but it saved a little bit of a hassle.
For every trip, I try to pack as if I were staying for half as long as I am, plus 2 days. And, then I do laundry while we're at the hotel for an afternoon park break. That saves a ton of space, but does require me to remember to do the laundry!