Sunscreen good. No sunscreen, bad.  
Rest of advice, based on years of Jedi-teaching experience, yes!

If Celestia gets sunscreen on her, will she die?

…or does the mere sight to …Cream bring out the Molestia in you, Celly?!
 
@igotnopicks
 
“Princess Celestia does not like Sun protection.”
 
…instead, she likes Mirror protection:
 

 
X3
 
@Lupin Quill
 
No, ’tis…:
 

 
…and as Twi is ‘Tia’s anointed one, and her Faithful Student, thus Twilight Christ.*
 
Fundie preacher: ‘Awl prrraise-ah Twahlaight! Prraise thuh Lady-uh!’

@Lupin Quill  
Power of Coppertone! The Power of Coppertone compels you!

Pffffhahahahaha! XD

Jeez. Sorry I offended you, Princess Melanoma.

@Lupin Quill @FanOfMostEverything  
Faust. the Power of Faust

@JP
 
Celestia was about to strike him down with all her fury…and then realized he had skin cancer from staying out in the sun too long. She backed off and apologized after that.

@Lupin Quill  
Which one? Cake seems like a good standby for Celestia.

Okay, so that’s parasols and sunscreen on Celestia’s lists of kryptonites.

Come on, Celly. What about all the ponies who go inside to stay out of the sun? Wait… what happened to the poor guy who invented sunscreen in the first place?

I like the idea that putting on sunscreen cuts off Celestia’s power.

THE POWER OF -  
Crap, what power compels a princess?

Princess Celestia does not like Sun protection.