I try to pack anything I think I might need. I just don't want to 1. Deal with the hassle of tracking down a needed item and 2. Pay an overinflated price.
That being said I do try to get away with the smallest bag/s possible (allowing for room to bring home things).
At Disney I generally choose not to go carry on only because I usually will not have a car and they do price gouge. I also know that I'll be bringing stuff like things to decorate my window and maybe a few extra shirts/layers because I always seem to go through more clothes there, and extra shoes because switching out shoes can save a vacation with that much walking. I also know that I will be bringing back stuff for myself and my mom who loves stuff from Disney.
Other locals I can totally get away with a carry on and my backpack.
I have a mini ipad (books, movies, music, internet, time killers all in one small device), I use my phone to take pictures as I'm not into photography as a hobby. I have a collection of packing cubes and utilize ziplocks heavily, between the two I can increase my usable room by at least half.
I am thinking of trying DLR with a carryon in April as I am may be doing a split stay on and off site so will have to transport my luggage mid trip. I won't need some items and I doubt I'll be bringing back much, so I'm already strategizing.