what level of humor resonates best with your players?
if your group is all young males, then using ghost sound to emulate whoopee cushions, placing illusionary stains where people were just sitting, having things jump out and say boo, and having a distractingly attractive female illusion standing in just the wrong spot to cause accidents, would be considered a fair start.
for the more complex and mature group, i guess it depends on if you are going to be cruel or cunning. watch some episodes of mission impossible, and look for where illusions would have (or did) come in handy. you could convince someone that they are going mad by twisting their perceptions subtly, bit by bit, especially if you can manage to limit the illusion to just them. illusions can cloak, and disguise, and mislead... and even hurt if they are believed in too much.
illusions can enhance a performance, in subtle or obvious ways. i recall one gamer who used illusions to put emoticons above her character's head, up to and including a little rain cloud when they were sad. in the words of one famous super villain - the difference between a villain, and a super villain, is presentation! or rather: PRESENTATION!
if you are going to focus on pranks, then the best is to mix reality with illusion, so that the observers become uncertain as to which is which. cloaking holes or doors, making fake openings and doors, fake snake in the sleeping bag, voices from items and backpacks, the possibilities are vast. i might advise picking up telekinesis when possible to pack an added punch.