I need to survive for 3 days while pooping as much as possible. I can pee as often as I like. It can take up loads of space. What food do I pack?

I have a good option here: Dairy.

Basically, your body can only produce so much Lactase (the thing that breaks down Lactose) at a time. Lactose intolerant people barely produce any Lactase, which is why eating a tiny bit of cheese is OK for them, but they'll explosively shit themselves if they have a glass of milk.

Anyways, you have a lactase limit, time to find it. An added bonus of this strategy is that your options are a bit more varied: You can load up on cheeses, milkshakes, cream puffs, cheesecake. Regular old milk has the most lactose though, so if you want to shit yourself plentifully and violently, bring a ton of that (and whatever other foods you want, the milk will do the work here). Just don't try and chug a whole gallon of milk in under 20 minutes, you'll vomit because it will dilute your stomach acid until instead of digesting it, it'll curdle in your stomach.