I agree about making costumes, even if you don't sew. I live in the northeast, as you do, so we always make stuff out of sweats. I hate seeing little kids in flimsy costumes and then wearing coats over them. You'll be in Florida this year so you need to go much lighter, but for future years, think sweat suits in any color to match the theme/critter you are trying to create. I agree wholeheartedly with those who say you give values to kids about not just popping off to the store for something, especially for a one-time use. And having them participate in the creation of a costume is both a bonding experience and a lesson in working for what you want.
Any good fabric or craft store will have people who can show you the latest in easy-to-use materials. There is fabric paint, sure. If that scares you, there is fabric glue and iron-on stuff that can even wash out. Felt is sold in long pieces and in squares, and can be cut with sharp scissors into pieces that require no sewing of the rough edges. There are simple pattern books to follow.
You have good suggestions already on the butterfly theme, assuming she sticks with it - store-bought wings are easy to find, headband antennas are everywhere, and light leotards and tights will have other uses. And they won't take up a lot of packing space, which is great for your needs this year.
For the future, think outside the box. One year, my kid was a robot - we cut a whole in a box bottom and inverted it over his head, with 2 holes for his arms. We put a smaller box over his head as a sort of helmet. We painted the outside with leftover white paint, bought white dryer hose which we cut into arms and legs (safety-pinning the ends inside the box). We blued a bunch of things on the front to be the buttons and dials - old TV remote controls, the rubber tips that go on the bottom of chair legs, anything else we found in the basement with no apparent use. He wore long underwear and sweats underneath, and was toasty warm. We even hung a cup hook on the outside to hold his trick-or-treat bag - he acted very "robotic" and the neighbors loved putting the candy & toys right in the bag.
I've seen people create a "mailbox" costume out of a larger box - the larger public mailboxes you see on street corners. The fold-down flap became the opening to the mailbox and a candy bag is attached inside. Basic blue paint and some white lettering for "U.S. Mail" and so on. Make a mistake? Paint over it!
There's so much stuff on the internet now. Take the time to look around. If you want inspiration, look at this costume the parents made for their child in a wheelchair - they used a big box behind his wheelchair with a 2nd set of wheels, and the wheelchair was the front wheels of an ice cream truck. Truly brilliant! http://www.huffingtonpost.com/2012/10/01/best-halloween-c...
The point is, with planning and Google, you can create fabulous things!