What are some cheap, last minute Halloween costume ideas?

I want to go as something that I could just wear normal clothes but have a twist with them. For example; going as a nerd or a geek, because all it involves is a button up with a tie. I have no time to go out and get a costume, so I was thinking of making something at my house. Costumes along the lines of a nerd, ninja..etc. I want something original. Be creative! Thank you very much for your ideas! :)