Have a very full steamy bath, splash around so 65% of it is on the floor and then, when challenged, shout "BUT I'M BEING A MERMAID"
Decide my name is : Elsa, Anna, Olaf, Charlotte, Florence or Beatrice but not tell anyone and then refuse to answer to anything else. Keep em guessing!
Develop random age-inappropriate obsessions, like oooh I dunno, the current PM, and ask to watch videos of that as a condition of going to bed, brushing teeth etc.
Refuse to eat, sleep, bathe unless I am holding a specific, totemic object with no instrinsic value, like an old coffee scoop, broken comb etc. Treat this object with gay abandon and unconcern all day - lose it at least 5 times - but then insist I cannot possibly go to the loo unless I am holding it.
Help clean their house by carefully washing all the powerpoints with a sopping wet rag that is in fact their best face cloth.
Totally ignore them when they come home but cling around their ankles when they are going out
, saying things like "why do you HAVE to go to work" and "I won't get dressed if you leave". Bonus points if they are already late. Recover as soon as they get out the door and wave cheerily from a window.
And then make it apparent it was really the binman or NDN cat I was waving at, as I have, in fact, already forgotten their existence.