The Cat's Adenoids

If you can't think of any good compliments, it turns out that you can combine any animal with any noun for an acceptable flattering phrase.

"...just about any combination of an animal, fish, or fowl with a part of the body or article of clothing that was inappropriate for it: the bee's knees, the snake's hips, the clam's garter, the eel's ankle, the elephant's instep, the tiger's spots, the leopard's stripes, the sardine's whiskers, the pig's wings." Other sources list "the kipper's knickers, the duck's quack, the gnat's elbow, the elephant's (fallen) arches, the bullfrog's beard, the canary's tusks, the cuckoo's chin, the butterfly's book, the caterpillar's kimono, the turtle's neck."

More info here.

Maurice Weseen's Dictionary of American Slang lists more "cat" variants: "the cat's eyebrow, ankle, adenoids, tonsils, galoshes, cufflinks, roller skates, and cradle."