My suggestion is a garden fairy! I was this a long time ago but it was my best costume!
What you need is a white dress, which should be kind of poofy! Then, you have to put a bunch of plastic flowers all over the dress, that you can buy at a craft store. With the flowers, also make a crown out of them.
That's really all you need. And for your makeup, wear white eye shadow, glitter on top of it, and put shimmery powder all over your face. Also put the glitter on your arms and neck.
And there you go! Have fun Trick Or Treating!
Thanks to Soccer Chic for this fun idea!