Prof. Filius Flitwick:
Sing a Muggle tune at a choir concert and start once again even after Flitwick scolds you. Promptly switch to singing a ghastly rendition of "Double Trouble"
Offer him an antidote to cure his dwarvism
Ask him what job he has at Gringott's. He yells that he's a dwarf. Say, "So you do admit it! I think you're going to need that potion now..."
Professor Flitwick has no doubt heard all the "swish and flick" jokes, but tell him some anyway (Keep in mind Madam Hooch has heard the 'broomstick" jokes too, but hey, that's your call)
Prof. Sybil Trelawney:
Bring a Magic 8 Ball to Divination (classic)
Tell her you're a Seer and prophesize that she has a dark fate in the near future, possibly involving the Grim and some unruly gnomes *shudder*
Prof. Minerva McGonagall:
Transform all her belongings into tabby cats, pocket watches, and water goblets. Say you're simply practicing your Transfiguration.
Surreptitiously replace her spectacles for aviators and her pointy hat for a fedora. Now we're talkin'
Give her a ball of yarn and Scabbers for her to play with
Other:
Introduce Alg, Geo, Trig, and Calc to the Arithmancy students
Abruptly start belting out "Lollipop" during the N.E.W.T.'s. When people start looking around, point to the nearest Hufflepuff; it's always them.