Celestia is pretty epic, and a lot of people like to poke fun at the royals of Canterlot. But still, like any other pony, they MUST dream of having days of total relaxation every now and then. Twilight becoming a princess was the catalyst for these thoughts in this case, and truly, since they're such good managers of the kingdom, they probably COULD have squeezed some fun into their schedule.
But at the end of the day we must all look deep within ourselves...and chortle uncontrollably at the picture of the two sisters going down a water slide on those bouncy-yoga-ball-thingies!