I need to figure out a costume! Help me pick...

I was planning to do a Star Trek: TOS science officer uniform, but I've been too busy to make one and just about every store that carries them only has sizes XS-M. I need a large. So that is out, especially since I don't want to pay overnight shipping on an overpriced, cheaply made costume that might not even fit right.

I have a few supplies in my closet/costumes that I can do from scratch. Last year, I wore a navy bedsheet as a toga with giant/chunky gold jewelry and a golden Grecian mask. I would rather not do it again, but I definitely could. I also have a gorgeous green hippie dress with trim and laces and a brocade pattern that I've worn ~3 different years in a Titania costume. Again, I've done that and would prefer not to repeat.

So this brings me to quick-and-easy-and-not-too-expensive costume ideas.

1. Louise Belcher. Recognizeable and pretty simple. I'd need to find an oversized green t-shirt or a green dress, and then make the bunny ears tomorrow. I probably could get this done.

2. Fionna from the gender-flipped Adventure Time episodes. Again, recognizable, but maybe it's overdone? The reference is a few years old at this point. I have the blue mini-skirt, so I would need a teal shirt, thigh-highs, and the bunny ears again. I could probably get the bunny ears at Hot Topic, which is pricey but convenient. I probably would leave out the backpack and sword, because time and monies.

Other easy ideas? I really don't want to spend a ton of money.