I've never entirely understood 'as safe as houses'. Yes, houses are meant to be stable and all that, but they do sometimes fall down. Guess it's just talking about as safe as can reasonably be expected.

How about "as cool as a cucumber" (was it here or somewhere else I read that it's been shown that the insides of cucumbers are always cooler than the ambient room temperature, at least until it gets stinking hot; that's way cool). And it has that lovely lilting alliteration.