Im a double majoring in math and physics, recently I have aquired a fair maiden that eats my so called "cummie wummies", plays vidya and likes to shoot the shit with me in general.

I want to give her something for christmas but since I'm kind of broke at the moment (I've only got 35 bucks for gifts) I was thinking of sending her a nice letter complete with drawings, a written story and if I'm not too lazy, some math food for thought like giving her the definition of open/ closed sets and asking her to find a set that's both open and closed in the Rationals or, alternatively, giving her the notion of what a Dedekind cut is.
She's clever but she's only a chemistry major, so I'm worried about sending her a challenge like that to try on her own (although we have talked about what a set is and Russells paradox). What things could I gift her that you'd think she'd like but are inexpensive?