So I am supposed to be writing my book but I'm writing this.

I was wondering what squibs could actually do, and I realised that the answer is actually quite a lot.

Astronomy
Well, you don't need magic to stick your eye in a telescope.

Herbology
You might need to know certain spells to be able to care for certain plants, but Harry never used magic in his lessons.

DADA
Well... if you have a toad for a teacher

But a lot of it is actually theory (signs of  a werewolf, dark artifacts, effects of curses e.t.c)

Potions
According to Snap there is 'no foolish wand waving' and I'm gonna take his word rather that jk's

Charms & Transfiguration
Sorry mate

History of Magic
Check

Care of Magical Creatures
Muggles can't see them, but I think squibs can.

Divination
You've got muggle diviners, and a lot of what harry got taught was interpreting what was already there, so not sure

Summary:
Astronomy YES
Charms NO
Potions YES
Transfiguration NO
History of Magic YES
DADA MOST OF IT
Quiditch YEA
Care of Magic Creatures YES
Divination MOST OF IT
Herbology YES

So all in all, squibs can do much more than you give them credit for, and it's another chapter in the massively flawed potterverse :)

-charlie