Let's do it, Lulu!
Although immortality was a wonderful gift, allowing her to serve her ponies for countless years to come, it did have its downsides. Namely, that immortality caused you to get bored very, very quickly. If you couldn't overcome that boredom, it would drive you mad, which was what happened to Discord. Thankfully, her parents had seen this coming a long ways away, and sent her and Luna to stop Discord before he ruined a perfectly good world to inhabit and guide the sentient beings there to peace and harmony.
If anything, however, Celestia swore that she would become the next Discord, and finish what he started if she didn't find something to occupy her time with. Sure, there were the latest trends that ponies seemed to like, but they simply felt like fads to her that would go away and fade into obscurity in the blink of an eye. There were things she did because of her boredom, such as reading (Faust forbid it would be anything related to taxes and bills, or those trashy novels about teen romance; where did all the writing creativity go?), skipping stones across a pond (her personal best was ten skips, though Luna could easily triple that in her sleep while drunk and being taped to the ceiling upside down with duct tape), and staring up at the ceiling of her bedroom, counting the tiles that made up her ceiling (she always lost count somewhere in the four thousands), but these were things that anypony could do, and those activities didn't really relieve her of her boredom; instead, they usually just made it worse for her.
Indeed, she was in her room, without a single thing that she hadn't done nearby. Read old friendship reports from her student? She already did that every weekend. Squeeze specially made stress balls while thinking about something stressful? She needed to order a new one after Luna had pranked her; she had gone to her room and squeezed the stress ball so hard that it popped under the tremendous force of her hooves. Look out the balcony? Tempting, since there was always the chance that something new could happen, and she hadn't looked actually been outside on her balcony for a long while.
Sighing, she took a moment to stretch her limbs out before lazily rolling off the side of her bed; despite the fact that she was the older sister that did most of the paperwork, Celestia was an incredibly lazy alicorn by alicorn standards. She once again savored the fact that the floor now had carpeting (The lucky carpenters that had furnished her room got paid twenty times the amount they would usually have been paid, much to their amazement and delight) for a moment before getting back up on her hooves to slowly walk over to the balcony. With a gentle push with some magic, courtesy of her horn, she pushed the doors that kept her balcony and room seperate, and stepped outside.
The first thing that immediately came to her mind was that she should probably reconsider the silencing spell that completely nullified any sound trying to enter her bedroom; if the spell wasn't in place, she probably would've known that it was downright pouring outside. Needless to say, Celestia was instantly soaked the moment she stepped outside. Undaunted by the sudden physical change from being warm and toasty to cold and wet (alicorns didn't get sick, at least not easily), she scanned the area for any points of interest; alas, there was nothing that kept Celestia's attention for more than five seconds.
Of course, any sentient being with a lick of common sense was in a nice, dry, warm place, and if they weren't, they were quickly heading there. Groaning in exasperation, Celestia walked back into her bedroom, and closed the balcony with a slam; today was the day that she would go mad like Discord, it seemed, unless her sister had anything for her to do.
Deciding that it was her last chance to NOT go crazy, Celestia quickly fried (yes, fried, not boiled) the water on her coat, effectively evaporating it in an instant and giving her a slightly fluffy look that a little grooming later would take care of; for now, however, she had to find her sister, and she prayed to Faust that she would have something that would interest her.
As she walked down the hallway to the lunar court, she couldn't help but notice that the ponies she ran into often gaped at her, probably from how she looked. Sighing at how impossible her ponies could be, she continued on her way to Luna's court, giving the lunar guards a curt nod before striding into her sister's court, drawing in her breath in an attempt to speak to her sister...
...Which died out as she saw how empty the courtroom was; Luna was playing checkers with one of her advisers while the rest were playing poker at a table, many of whom had extremely poor poker faces. The guards were yawning, and some were even slumped against walls, taking a nap on duty. All in all, it was quiet, peaceful, and completely different than the solar court, which was always packed, chaotic, and full of ponies asking her questions or shoving proposals for the 'benefit' of Equestria in her face. Mentally, she made a note for half of her current court to seek audience with Luna; she could handle some of her ponies if she had the free time to play checkers with one of her advisers!
"...Ahem."
The clearing of her throat instantly caused a chaotic uproar, seen by guards fumbling to wake up and stand at attention rather raggedly, the advisers playing poker to suddenly overturn the table in their haste to run over towards Luna in an attempt to 'advise' her, and Luna herself giving a surprised look and a suppressed snicker at Celestia before looking around at her courtroom and smiling at her sister sheepishly; silence reigned over the courtroom for a few moments before Luna dared to speak.
"...Hello, Tia! What brings you here to my court?"
"I need to talk with you about an important matter, Lulu. Please, dismiss the ponies in your court here."
Celestia didn't miss the flicker of uncertainty that flashed in Luna's eyes before her sister gave a slight nod, causing the ponies in the courtroom to leave quietly. Some of the braver ponies chanced a glance at Celestia, who was maintaining her usual composure, despite the fact that it just looked like she had just climbed out of a dryer. Once all the ponies had exited the courtroom and the doors closed, Luna burst out in laughter, and Celestia scowled as she strode over to Luna to clobber Luna with a hoof on top of her head.
"Ow! Hee hee, sister, what happened to you?!"
"That's not important! I'm bored, Lulu! I need something to do!"
Luna's laughter ceased, and she turned slightly pale as Celestia said that she was bored; this was terrible! She gaped at Celestia, who had a mournful look on her face.
"Tia! This will not do! You remember what happened to Discord; we...I cannot allow it to happen to you as well!"
"I know, Lulu! I don't know what to do, though! I can't think of anything, and you're my last hope, Lulu!"
Pacing around Celestia agitatedly, Luna tried to think of things that her sister in her right mind would not have considered doing; one of her sister's few weaknesses beside bananas and spiders and other such things were her inability to cope with boredom; in fact, she wondered how her sister even managed a thousand years by herself! She shot suggestions at Celestia, who turned down or made up an excuse for all of them but one.
"Have you sparred with your guards?"
"The guards deem it too dangerous for me."
"Help the gardeners in the Royal gardens?"
"They don't want me to get dirty."
"Blow up a mountain?"
"Why?"
"Wave your fat flank at Discord's statue and laugh at him?
"That's just rude and indecent! I am also not fat!"
"Play video games?"
"The only pony I can ask to play with is with you, and you almost always win."
"Streak across the castle?"
"...Luna, we don't normally wear clothes anyway, and that might cause a political incident."
"Visit your student?"
"I'm not able to do that frequently enough."
"Oh, oh, I know, let's prank your student, Twilight Sparkle!"
Now, that was an idea that had never crossed her mind; for how often she got bored, she had never once considered pranking her student...Or anypony, for that matter, despite the fact that she liked to tease other ponies. She looked at Luna with a hopeful expression on her face that it would have killed any other pony, were they not an alicorn.
"I...Have never considered that, Lulu. Do you really think that will help me fix my boredom, Lulu?"
"Of course! Now, all we need is a stapler, a gallon of milk, some fish, a box of raisin bran, and a frying pan!"
Celestia clapped her hooves together in excitement before hugging her sister tightly, causing her sister to wince from the force of the hug.
"Yay! Let's do it, Lulu!"
"Celestialetgopleaseyou'recrushingmeithurtsreallybadlyFaustpleasehelpyourdaughter."