When they ask me in advance if there is anything in particular I'd like for meals/ask me for input into a weekly food plan I will make it clear that I'm too busy for such trivia and tell them that I don't mind, I'll eat anything, they should just get whatever they want to cook. When I am there, at each mealtime I will ask what we're having and pull a face, ask if there's anything else, then when I'm told no rummage through the freezer and find something I would rather have. This will always be either convenience food bought for a night when we won't have a lot of time, or a random selection of foods that are actually supposed to be components of a meal later in the week. If they insist on sticking to their meal plan I will eat 2 mouthfuls before declaring myself full- then eat sneak crisps and biscuits up to my room half an hour later.
When I sneak crisps, biscuits or other snack foods I will, of course, be sure to leave the outer packaging in the cupboard so it looks like none have been taken. When I am given permission to have one, but I have actually eaten them all already, I will swear profusely as I take out the empty packaging and ask why we never have any decent food in the house.
I will also complain that they never buy cans of soft drinks, but when they do I will take a can approximately every hour that I am in the house, have a few mouthfuls then leave it on a windowsill to go flat.
To be honest, my MIL also does half of this when she visits so perhaps she is also getting her revenge on DH.