Unfortunately, Rarity was in this episode. Just kidding! There was only a little Rarity, and she's fine in small doses.
The plot is: Pinkie Pie overhears a secret and is terrible at keeping it. And so are the other ponies that also know the secret! She's also been making way too many Pinkie Promises, so she can't just hide in her room.
Her so-called friends react by telling her that nopony should ever, ever ruin a surprise, and then chanting 'tell tell tell!' at her until she almost ruins the surprise. Although the last part was semi-imaginary.
Ninja Pinkie was funny. I bet Rarity ordered that cake just so that she could get Pinkie Pie alone -- FOILED! And the CMC are now bored out of their minds since they already have cutie marks.
Oh look! The infamous adult blank flank Candy Mane finally got her cutie mark! I just wish she wasn't soooo far in the background that you can't see what it is. Maybe a box with gears floating over it? Just a bunch of flowers?
The moral of the story is that people shouldn't rely on not being told spoilers, because it's really painful for everyone who knows and it doesn't matter that much anyway. Or possibly the opposite, but I'm going with my gut.
But it was a fun episode, without anything to really drag it down. I'm going to provisionally give it 3/3 although I'm worried that might just be an overreaction to the last couple episodes being terrible.