Ok, this is swimming against the tide, but here's my advice--don't underpack! Everywhere your read for travel advice (to Disney and elsewhere) this seems to be standard advice, that is DON'T OVERPACK. Well, there is some merit to this, but I'd rather take a few things I don't need rather than wish I had them. After all, you will be living for maybe a week or more in a hotel room. To think you can buy what you forget is not very realistic. If you don't have a car you're at the mercy of whatever overpriced hotel shop you can find. Even with a car, you can burn half a day running around looking for something you could have easily packed. Not to mention the cost.
Everytime I've packed light I've regretted it. One summer we went to San Antonio and took just enough clothes for the days we'd be gone. Well, I failed to realize how hot it would be. We needed to change clothes at least 2x day, maybe more. We ended up washing out clothes in the bathtub with hotel shampoo (I didn't pack laundry soap either). What a mess! Plus, we smelled like the shampoo! Another time we went to Las Vegas with the absolute minimum. We decided to extend our stay, and I ended up buying 2 tee shirts at the gift shop for $35 each--$70 to walk around with hotel Mirage's logo on me--they should have paid me! Another time we went to San Francisco and I didn't realize how cold it can get there during the summer. There again, $40 for a sweatshirt with someone's logo on it. Do plan your wardrobe for contigencies such as changes in weather, extra days, soiled, etc. We were just lucky we could find what we needed at any price! And do be sure to take along laundry soap--trust me, you'll use it. And also a Shout stain stick.
Also, like someone else suggested, take along a "medicine chest". Just use a ziploc bag and take the pills in blister packs, not the liquid. My suggestions--Tylenol. Motrin, Pepto Bismol, Benedryl, sinus/decongestant, your favorite cold remedy, and last but not least, DON'T forget the Inmodium!! Your can imagine the prediciment you'd be in for lack of this. It takes no room to carry the pills. Even if you think you won't need it, well, just the stress of travel, change in routine, different food, even local water can all cause this. Once I was travelling alone in Toronto. I got such a bad case of diarrhea and stomach cramps I couldn't leave the room. I called the gift shop. They had some Inmodium and brought it to my room. Needless to say, this little act of kindness was to everyone's advantage. Also, I tipped well. But all that could have been avoided with a little pre-planning.
Every time I travel I always remark when we return that we didn't use half of what we brought. But, as my husband says, how did we know which we'd need until we got there? Of course, if you're going to be on the road, every night a different place, constantly on and off trains,planes, buses, etc. you need to eliminate as much weight as possible. However, if you'll just be staying in one place for the duration, such as Disney, if you think you might need it, take it!