i like most of the slim cases on Apple's
website (you might have to refine the search by selecting "iPod nano (aluminum) from the drop-down menu).
link to UK store - the selection is greater in the US.
Radtech also makes a silicone case, called
Gelz, and a sleeve, called
Sleevz (both links to US store; i'm not sure if Radtech has a UK store).
from what i've seen, the aluminum nanos are pretty resilient so i would guess even an iPod sock would work.