A cat may look at a king: all people are equal, irrespective of their social background.
“His arrogant manner with the staff is highly objectionable; he should remember that a cat may look at a king!”
Cat’s pyjamas/whiskers: the best.
“My new car is just the cat’s whiskers.”
Fight like cat and dog: be continually quarrelling.
“There’s never any peace in our house because my two daughters fight like cat and dog.”
Let the cat out of bed: unintentionally reveal a secret.
“We were going to throw a surprise party for Tom’s birthday but Gill let the cat out of the bag and he knew all about it beforehand.
When the cat’s away the mice will play: people will take advantage of any lack of supervision.
“Colin has been to the pub every night since his wife’s been in hospital; still, when the cat’s away!”
(From John O.E. Clark, Dictionary of English Idioms, Harrap’s)