Cut the oranges in half and scrape out the pulp, making sure you don't break the rinds. (A grapefruit knife makes this an easy job.)
Slice off the very tip of the base of each orange rind, if necessary, so it sits flat and forms a cup. Be careful that you don't cut too much, as you don't want to make a hole at the bottom. Set aside.
Prep the other ingredients and add to a large bowl. Set aside.
Mash the reserved orange pulp, cut it with a pair of kitchen scissors, or press it through a fine-mesh sieve if you prefer just the juice, then toss it with the other ingredients.
Spoon the fruit salad into the orange cups, then dollop 1 spoonful of yogurt on top of each fruit salad.
Serve immediately or reserve for up to 2 hours. Know that the banana and apples may brown slightly as they sit.
Notes
What makes these fruit salad cups super-fun is that they're served right in the orange rind of the orange that's cut up for the salad. However, if you don't want to take the time to scrape out the orange rinds OR you want to send these in a school lunch, just skip the orange rind bowl and place in a leak-proof container or serving bowl. It's simpler and much, much quicker. ?