How would I teach the concept of opportunity cost to 5th graders as a story?

I suppose the framing of the story would depend upon the outcome you are hoping to achieve and the assignment that would follow, but I would set it up as a situation where there will not be an obvious choice. Instead of demonstrating something like play outside vs. doing chores, propose something that involves the same gratification up front, but different benefits long-term. Example: Your mom asks you what you want for your birthday. You're turning 10, double digits - it's a big deal! She says you can ask for anything you want! You think about it for a long time. You say you want a puppy and a big birthday party with all of your friends. She says both things are great, but you can only choose one. The puppy will be your friend for years! But a big party with your friends would be super fun! Is it better to have a furry friend or a super fun day with friends? Both options have upsides, but choosing only one offers a downside either way. The potential to gain popularity at that age is important.

Please see attached reading and Q&A for an activity that asks the student, "Should LeBron James Mow His Own Lawn?". It defines and teaches them about Opportunity Cost as well as Comparative Advantage and Absolute Advantage. Numbers 4 & 5 on the Q&A worksheet ask the student to write a short response (could be turned into a story) to check for understanding.

Sam is in the fifth grade, and he has a decision to make.
Sarah, a girl he really, really likes, has invited him to her birthday party next weekend. It would be the first time he has had an opportunity to spend time with Sarah outside of school, and the party sounds like it will be a lot of fun.
Sam's father has presented Sam with a different opportunity: the chance to attend a professional football game involving his favorite team. He's never been to a game before, and it sounds like a great opportunity. Unfortunately for Sam, the game will happen the same weekend, and at the same time, as Sarah's party.
If you were Sam, which event would you pass up, and why?

In trying to create a story for fifth graders, I would try to find something that related to them. Opportunity cost can be defined as what is given up or gained as a choice is made. For example, in the case of the fifth graders, I would ask them what they could possibly do on a Saturday afternoon. They could do chores—an activity that would make their parents happy. They could study in order to get better grades. They could spend time with friends. They could exercise and be healthy. They could also play video games or watch television. For the story I would create, I would have an imaginary fifth grader make a choice as to what he/she wanted to do on a Saturday afternoon. I would also have this same fifth grader examine what is given up by making the choice. For example, the fifth grader could relax by watching cartoons but have parents who might not be happy due to chores not being finished. The fifth grader could also lose out on spending time with friends or have lower grades due to not studying. The most important things are that the story is something the students can relate to and it demonstrates opportunity costs. The students should see that they deal with opportunity costs in their everyday lives whether or not they know that they exist.