Just don't buy anything from the concession stand. You CAN sit there for a
couple of hours without munching on anything.
Bring your own snacks (popcorn, candy, beverage in plastic bottle [can't
bring glass into the theater], sunflower seeds, nuts, sandwiches... even
hitting a fast food restaurant on the way for a meal deal). Most people assume
this isn't allowed and there are theater employees that will try to tell
you as much; but the reality if you insist, they can't stop you bringing
your food with you UNLESS THERE ARE CONTAINER ISSUES.
You could also go to the dollar tree before the movie.