1. Text replacement
    Settings > General > Keyboard > Text Replacement. You can set any word to autocorrect to any other word or phrase. Best prank I know of.
  2. Selfie as wallpaper
  3. Sending Venmo requests to people for bizarre amounts & reasons.
  4. Renaming their folders with creepy messages
  5. Switch "mom" contact to "dad" and visa versa.
    Very subtle but will probably lead to mass confusion. Be sure to swap photos, too.
  6. Write on Applebees' Facebook wall. They always reply.
    This one is courtesy of @ErinFlaherty back when we were first roommates.
    Suggested by @sky
  7. Switch their crush's name with your number, text them as their crush.
    Suggested by @supabg
  8. Write a list ! DUH
    Suggested by @Lisa_Fav
  9. Change qwerty keyboard to azerty one
    Very discreet yet terrible one
    Suggested by @Lisa_Fav
  10. Every time I left my phone available to my kid, this thing showed up everywhere. Screensaver, profile pic, texts to randos, etc.
    Suggested by @clairewentthere
  11. Make a status update that's strange but not over the top so it's still somewhat believable. This is my all time favorite status @sky has made for me.
    Suggested by @ErinFlaherty
  12. Switch their iPhone's language to Cantonese
    (Fun fact: that's what @Flurn used to do to the West Wing cast)
    Suggested by @bookishclaire
  13. Join really strange and obscure Facebook groups.
    They won't know which ones you've joined and will keep getting updates for a while until they've left all those groups. The gift that keeps giving
    Suggested by @estherlimtf
  14. Whenever my college roommate stole a friend's phone (usually mine) she would text "I have to poop" to our other friends.
    She'll still do it occasionally and it still makes me laugh.
    Suggested by @supercommonname
  15. Switch to "edit" mode (where all the apps have x's on them) and take a screen-shot. Make that the wallpaper and put all their apps in a folder on the second page. Even if they notice the apps aren't "dancing", it will still take a while to figure out what's going on.
    Suggested by @harmzie
  16. Straight up just Venmo yourself like $30
    Suggested by @graceadilla
  17. Add a nickname to your contact name, like 'Your Bae', 'World's Greatest Wife/Husband'
    Suggested by @THEToughCookie