In addition to being an astronaut, Scott Kelly is one-half of a pair of twins and a lab rat for NASA researchers studying the effects space travel on the human body. This NPR story details how NASA has been using twin research in order to learn more about the side-effects of prolonged time in space as the agency prepares to go to Mars.
Scott and his twin, Mark (also an astronaut!), have been providing all manner of bio data to researchers. In particular, researchers are interested in the effects of weightlessness as well as exposure to space radiation on aging.
This story provides a good example in class, as you can discuss twin AND longitudinal research. I think you could also use this example to introduce the concept of paired t-tests.
Preliminary research is available if you want to flesh out this example.