I like the approach of a small, fun task as "extra credit". But how do you smoothly bring that up in the interview? I guess if you're being asked about CSS technical details, you can mention you built this last night and ask if they'd like to see it. But I'm pretty sure due to nerves, I would make that really awkward. Like I'd answer the phone and shout "LOOK AT WHAT I MADE!"
B. Bring it up when asked about previous experience tangential to it
C. Bring it up when asked about your outside work activities or what sort of continuing professional development you've been doing lately
D. Mention it when asked about the "coolest/most fun" thing you've hacked on lately
E. Whip it out when asked the usual "what do you think we do here and what can you bring to the table"
Or, when you start asking your questions about the company/culture/ect. (DEFINITELY ask those types of questions), weave it in with a question about what kind of side-project or 20% type program they offer/support.
You could also throw it up on a design site or your blog and drop it in whichever social media channel they may have.
Cultivating a curious attitude (or sincerely making an effort to fake it) will help calm your nerves a bit as well.
