a few sandwiches short of a picnic
Someone who thinks a picnic is just a fancy lunch and brings a sandwich that looks like it was stolen from a vending machine.
My friend showed up with a sandwich that had three layers of bread and no idea what it was.
My uncle brought a sandwich that looked like it had been sitting in a hot car since the 80s.
My neighbor showed up with a sandwich and a napkin. He called it a full meal.