If you don't yet know the show, I invite you to watch the following promotional video.
Seriously, it is genius.
Tahmoh Penikett as Paul Ballard, Enver Gjokaj as Victor, Eliza Dushku as Echo, Dichen Lachman as Sierra, Fran Kranz as Topher Brink, Olivia Williams as Adelle DeWitt, and Harry Lennix as Boyd Langton, Dollhouse (2009)
Now, costume wise, say that you have a last minute Geek* (I mean Sci Fi) convention and you don't have anything to wear, you can simply grab your yoga training clothes, pick a name from NATO phonetic alphabet and go around telling people that you try to be your best. You can't get any easier than that!
*I consider myself to be a Geek. In fact, I score 39.05325% (Major Geek) on the Geek Test.