In regards to extra towels, I always grab a few of those $1 store towels that are packed small but expand when they get wet (really washcloths, lol) for the backpack, those have been great when we needed a towel but not a huge one. I also always pack each of the girls and I a Turbie Twist towel for the room. We all have long hair and like to have our hair up in a towel after we get out of the shower and the Turbie Twist saves towel usage.
Another thing I always pack are samples packs. I get a lot of different kinds of samples for shampoos & conditioners, lotions, cereals, energy bars, Tide pods, etc. Free is GOOD.
Oh, and depending on when we are going...a Snuggy (or 2). Perfect for when you are watching the movie at the resort at night, taking a nap in the day without turning down the blankets, etc. Folds up small, and there has never been once we had it that we didn't use it, but there have been times when we didn't have one we wish we did.