well, if you're looking for Disney related costumes, you could all go as princesses? That would be cute.
I made my own Tink costume last year.
All I did was buy a little white dress from Wal-Mart, then aI bought some sheer sparkly lime green fabric from a fabric store, some cheapy wings from a costumes store. And voila.
here's a picture;
Some other cool ideas...
a pumpkin (sounds cheese) but if you buy one that's made for kids and it's really short on you and you wear stockings with it, it looks really cute on a teenaged girl.
try going to this thread;
http://www.disboards.com/showthread.php?t=854129&page=16
there are some AWESOME costume ideas there!!!