Saturday we are supposed to hit a museum, I'm going to eat right before she gets here so I won't be hungry all day. I'm going to wait until she is so hungry she suggests getting something to eat, then technically it would be her idea, plus I'm going to give her the where are you taking me to line.
I can't fault your reasoning or strategy, but I think this is passive-aggressive.
Her attitude is not a small thing: that she expects, or will accept, a subsidy will be material to your P&L for as long as you know her.
I think it is better to take the opportunity to state your feelings, and get her's.