I like the idea of a Rainbow Dash/Rarity episode. The two haven't had much interaction in the past, and it could work well like Bats! did with Applejack/Fluttershy. Maybe RD would try to convince Rarity to play a sport, and Rarity tries to get RD to come with her to the Spa. They both are hesitant at first, but eventually find themselves enjoying it. They learn that you shouldn't hate on things until you try them