Hi Ben's Mommy!
Here are some things that made the trip easier on us, and DD:
* We brought our own stroller so she was completely comfortable. It has a full recline so she took most of her naps in the stroller and we never went back to the hotel mid-day. Since the parks weren't open late in the evenings, this worked out very well. I also got a mesh stroller bag from Babies R Us (made by The First Years) that attached to the handle. It was great because our stroller is designed so that everything falls out the basket when it's folded. So, we kept things in this bag and didn't have to take them out when folding for bus transportation.
* Bring some food/snacks along. I had a real hard time finding good food for her to eat in the parks. Most restaurants had a couple kids meal offerings for $3.99. We did that a few times, but not for every meal. It's kind of expensive considering she doesn't eat the whole thing. Plus, they weren't the most nutritious meals. We used Florida Towncars as our transportation to/from the airport which included a grocery store stop so I could pick up fresh produce and snacks for her to eat.
*Tabletoppers - Someone posted this on these boards before and we use them every time we go out to eat. I think they are especially important at WDW since it can be a pretty germy place. We also kept a tub of Wet Ones in the stroller to wipe hands.
* We rented a crib from A Baby's Best Friend and that worked great.
* This was our first flight with DD so I was very nervous. I brought a ton of snacks and some new toys/books that she hadn't played with before. One of her favorites was a bag of small slinky toys that I got at a party supply store. They are sold as party favors and they were just the right size for her to play with.
*Make time for the parades and shows. DD's absolute favorite event was Spectromagic and the Share a Dream parade at MK. We got a spot on Main Street about an hour before the show and ate lunch on the street so she'd have a front row spot. It was worth the admission price for her. She loved it.
*Baby Care Centers - We looked for these on the map as soon as we got to the parks. We really utilized these centers for diaper changes and nursing because they were so nice and quiet. The public restrooms were usually pretty crazy and I felt rushed.
*Baby Swap worked great on the rides.
I'll post more if I think of anything. Pretty much everything on this list is based on stuff I read from this forum.
You'll have such a wonderful time. Your 15 month old is going to do great!