The ex wants them, I don’t think my gf cares about them, but now I want them so the ex can’t have them.

Just take them.

I'd say that should be a matter for your GF and her ex.

Ughh you always giving me Dad advice when I wanna be petty

Way too obvious. I want her to not realize they’re missing until it’s too late. Or maybe she goes to unpack them and finds that a cup of water had spilled into the box and everything is ruined. I just want to ruin one of her days.

Just do the right thing. Better than falling into a downward spiral of trust issues.

Put googly eyes on the covers.