Well if you already have a costume you've used for a previous convention, I'd say just reuse it. (Some people find reusing costumes annoying, but it's better than rushing a whole new costume xD)
Otherwise, look at what you have around the house and pick a character with a simple outfit that you could modify or work off of materials you already have. If you're short on fabric/dont want to go out and buy alot of new stuff, you can always hack up old clothes and use them for material.
A friend of mine tossed together an Advent Children Cloud costume together in one night for a costume party using clothes she had lying around the house, and it looked pretty awesome for a 1-day job!
Can't think of much else to say without knowing what kind of characters you'd want to cosplay. xD