I spent the day playing around with mockups and compositions, but they were lacking... something. I realized that that 'something' is, first of all, game design and not art design, and I need to acknowledge that when I'm doing composition-work I can still count that as a success if I don't solve game design problems... but, that aside, I think what I was missing game design-wise was a locked door.
Maybe I should make a pattern out of this...
For now, I'll just say that in particular, a locked door represents an inviting and simple and impossible challenge: without the key, you cannot open a locked door. But you can! You can force it open, you can pick the lock, you can find another way around. You can also do some things with a locked door even if you can't open it. You can look at it and admire the outer surface. You can jiggle the handle. The space behind the door is implied but invisible -- except maybe you can peek through the keyhole to catch an unsatisfying glimpse of what's on the other side.
A locked door is really just a win condition.