Ask her if her relatives are half-breeds, because there's a resemblance
Keep time-detonating fireworks- courtesy of Fred and George -in her office
Ask Umbridge why Ravenclaw's symbol is an eagle, and not a raven. Ask more irritating questions like this until class is over
Inform Umbridge pink isn't her color and that when she gets older (than she already is), she'll be alone and pathetically living with cats
Persuade Trelawney into getting revenge on Umbridge. Have her predict (SPOILER ALERT! FORESHADOWING!) she'll be captured by centaurs and manhandled by a giant.