I have to agree, dollar store ponchos suck. You may as well just wear garbage bags. They stick to you, and they're uncomfortable. Once you take it off you won't ever want to put it back on, because it'll just be a wet mess of plastic. It's better to just buy one in the parks
Some airlines restrict the amount of checked luggage you can have. I would completely skip saving room for souvenirs or bringing an extra case for them - we just shipped ours home our last day there. Sure, you have to pay a small shipping fee, and you don't get it for about a week depending on where you live, but it's easier than lugging a whole extra suitcase just for souvenirs. Unless your family packs really light. Mine doesn't - for a 10 day trip, 4 of us had huge, huge suitcases (we had one extra person coming along). The only one able to use a smaller one was my 9 year old sister, and she still used the one I usually take for 5 day trips, it's still pretty big.
The shoe rack thing is a wonderful idea. Went to WDW with all my stuff in a makeup case with tons of compartments and zippers and it drove me crazy.
Bring an extra pair of sunglasses. I had mine hanging on a mesh call phone holder on the strap of my one strap backpack in a resturant, and forgot about it. Boy was I mad.
Pack for all kinds of weather if you're going in January... temperatures went down to a record low overnight when we were there, and boy was I glad that I'd brought 4 sweaters/sweatshirts. I got outside and ran back in to put one on over my shirt. Once we got to the park, I put on the second one I always had in my bag, and my jacket made an excellent windsheild while on the tram from the parking lot.
(Hear anyone yelling at the tram operator to get a move on on a freezing day at MGM during January? That would be my party. We sat there for like, 10 minutes and we're impatient. Not to mention I was probably the only one who thought to actually BRING my sweatshirts in my bag. Another reason NOT to use a fanny pack)
In the parks, most of us used fanny packs (7 out of 10). I like to bring a backpack to put extra sweatshirts in, and any souvenirs I might pick up. People may say that lugging around a backpack is a hassle - not really. You just wear it on your back and forget about it, and rides have a compartment to stow it in. It only takes three seconds to take it off and stick it under your seat. If it has no place to put it, just put it at your feet or hold onto it. It's really not that hard.
And last, but not least, MAKE SURE YOU HAVE EVERYTHING BEFORE LEAVING YOUR HOTEL/CONDO/RENTAL CAR. Check and double check every drawer, under beds, cabinets, tables, fridges, showers, shake out the bedding, everything. This last trip I left my cd case there - very disappointing. I'd brought all my favorite cd's for the 9 hour ride to and from FL, and I came home with the one inside my diskman. It wasn't in the box we shipped home, either. We called the condos we stayed at, the hotel, and the rental car place, and never got it back.