She's sadistic, disloyal and in desperate need of voice coaching- what isn't there to hate about Dolores Umbridge? It's simple: Fred and George didn't take it far enough!
Send a pack of frogspawn to her with a note attached: "I think these are yours."
Invite her to a Bonfire Night fireworks party. Casually mention at the last minute that the Weasley twins are running the show.
Set her up on a blind date with Firenze
Put a portable swamp in her study. When she complains, shrug and say: "you said you wanted it more like home."
Make retching noises whenever she tries to speak.
Tell her she and Argus Filch are made for each other.
Put a Galleon behind each of her china plates, then send a Niffler loose.
Tell her slime green suits her so much better than pink.
When she talks about her loyalty to Fudge (or whoever is Minister at the time) tut at her and say: "now now Dolores you mustn't tell lies."
Tell her she's like that Muggle woman off Keeping up Appearances- but worse.