All you'll need is a black dress with a white collar or a black dress and put a white collar under it. Then you throw your hair in braids.
2. Any Animal
Animal costumes are easy cause it just head to toe the fur/skin color add a tail and head piece and done.
3. Alice from Alice and Wonderland
For Alice all you need is a blue dress, white apron, and a black bow. If you don't have a blue dress with a collar, just put the same blue colored collared shirt under your dress.
4. Masquerade Girl
All you need for this costume is a mask and a dress. You pick.
5. Wind Up Doll Costume
The only "hard" thing about this costume is the wind up key. But you can easily make it out of toilet paper rolls and foil or even just painted cardboard. Then wear anything that reminds you of an old-timey doll and put some doll like makeup on.
There are many other costumes to be and many other easy ones. Like the obvious witch, mime, robber, dead person and many others. These are just some that I thought of. For example, the group I hang out with at school are all being Adventure Time characters. I am going to be Jake and Gaby is going to be Princess Bubblegum.
What are you guys going to be?