extension ideas, likely not all feasible:

[1] two Pis AIs play checkers/draughts against each other while a third watches and commentates vivaciously with pre-recorded commentary bits like they do for football videogames

[2] Pi uses microphone to listen to music and works out the time signature

[3] fridge magnet Pi that reacts to the fridge door movement and does something cute e.g. shows animations on a screen or says hi to you or something. maybe could display fridge temp. too
BUT THE FRIDGE MAGNET IS ACTUALLY A DISGUISED FRIDGE SECURITY SYSTEM:
Takes a photo of the fridge opener every time the fridge is opened, and the contents of the fridge at the point just before closing (has to be the point just before closing because at the closing moment the light inside the fridge turns off). It then uploads these photos somewhere so you can use them to work out who's stealing what food. someone keeps using my milk

[4] Pi with a camera and a laser pointer that reacts to a cat's movement and moves the laser pointer away from the cat. i dont think it's feasible but it's super fun

[5] Pi attached to a camera or some kinda sensor, and when you point it at something it tells you what colour it is. Tells you either in hex or in the nearest very specific colour name (bondi blue, shamrock green, goldenrod yellow, etc.), possibly by comparing with lists like:
http://math.ubbcluj.ro/~sberinde/wingraph/main.html#colornames
which already have associated hex RGBs, so that takes out a lot of tedium.
ALSO it could tell you the nearest Crayola crayon colour:
https://en.wikipedia.org/wiki/List_of_Crayola_crayon_colors

[6] Pi with multiple fingertip heartrate sensors attached (things that clip lightly onto your finger, and measure heartrate). You watch a horror film with several people and the Pi knows everyone's heartrates. When someone's heartrates begins to peak (e.g. at a particularly scary point in the film) the fingertip thing jumps a bit, or delivers an electric shock, to optimally scare that person. same concept applies to rollercoasters too, maybe?

[7] remote control rubber duck for a bathtub. water/electricity combo probably makes this unfeasible