Get a cardboard box and attach some lids on the front. Paint the entire thing with bright yellow , slip into it and voila...you are a Lego.
My friends call me a dork for my interests, I don't know you, so I wont suggest what I like personally. ;) Witches are easy... You could also be Mavis from Hotel Transylvania, if you've ever watched that movie... (My sister is going to be Mavis this year) You could be Marceline from Adventure Time :D and if you haven't seen Adventure Time, just be a regular vampire. When I was a kid, my mom used to make us these really amazing outfits x3
If you're a girly girl (which is okay, I'm not a girly girl, but I'll help) you could be a cheerleader, or a fairy (If you like Legend of Zelda, be Navi!! :D) You can be Jack Frost from Rise of the Guardians (If you don't mom genderbending) You can dress as a punk rocker, Uhmm... I feel I'm just rambling now :c I'll stop listing (tho I have more ideas)
A few years ago, I simply put on a winter coat, a Santa hat, and sang Christmas carols for Halloween. I mean it was weird but people appreciated my supposed Christmas caroler costume.
And then there was once where I was a laundry basket and I did that by cutting out a hole in an old laundry basket and squeezing myself through it. Then I just put some "laundry" (simply random pieces of clothing) in the basket, over the shoulder, etc.
I don't know, I usually don't dress up for Halloween so those were just a few things I've done once since I'm usually pretty lazy about it when I did dress up.