The person I'd choose is my cousin Joy, who works as a marine biologist in Palawan, in the Philippines. I work in a call centre in Manila, so our lives are very different, and I'll admit I'm a little envious of hers.
Her life is mostly about the sea. She's part of a team that monitors coral reefs and sea turtles in a protected area. On a typical day, she goes out by boat early, dives to check the condition of the reefs, and records how many fish and turtles they see. In the afternoons she writes reports, and in the evenings she sometimes talks to local fishermen about protecting the area.
If I could swap lives with her for a day, I'd want it to be a diving day. I'd love to get up at sunrise, have breakfast on the beach, then go out on the boat and see the reef the way she does, not as a tourist but as someone who understands what she's looking at. I'd like to help tag a turtle, which she says is the best part of her job. Then in the evening, I'd go to one of the meetings with the fishermen, just to see how she persuades them.
I'd like to swap partly because my own job is very indoor and repetitive, and I spend most of the day in front of a screen with a headset on. But it's more than escaping. I'd want to know what it feels like to do work that you can actually see making a difference. Of course, I'm sure her job has boring days too, but for one day, I think it would be amazing.