StickyJots provide a low-tech way for you to brainstorm iOS app interfaces. Dave Caolo asked me the other day whether we had covered them before and I was pretty sure we had not. These pretty kits start at US$20 for the faux iPhone, $35 for the tablet and $50 for the "everything kit."
Sure, they're not cheap, but they are quite lovely, and I bet they'd make a great gift for the developer in your life. Plus, they might even help you design the next great app.
Further details and an online store are found at the StickyJots site.