Well if you're set on going as Penny, you have 2 real choices.. cute shorts and a tank top (with pigtails or ponytails to suit) to reflect her casual look, or a ruched side bodycon dress (with hair down) and a pair of gorgeous matching heels. (match these with a cute clutch bag) to reflect her going out side.
As Suzie says, Amy or even Bernadette would be a "less obvious" choice, but less fun. If your intention is to stay firmly in the wardrobe, then doing any of these well with makeup and wig etc will jeopardise that, th eonly way is to play it as an obvious "joke" drag outfit. outrageous makeup and deliberately ****ty look, but let's be honest, you don't want to look like some outrageous drag act, you want to look as fabulous as possible.
If it were me, I'd just go for it, and get a close friend (who knows about Jessica) to pretend they helped you with makeup and clothes etc so that you can always laugh it off as just a party thing if you want to.
You could also always go for the wonder woman outfit that Penny wore for the new year party at the comic book store, which would be more of a party outfit than going as "just" Penny.
