The scenario here is basically my own epilogue of the episode “Dragon Dropped” (which is hated by a lot of MLP fans due mostly to the portrayals of Rarity and Gabby the Griffin and feeling insulting to Sparity shippers):
A little time after exploring the gem cave (the bats thankfully didn’t stop them), Pinkie Pie has gone home to Sugarcube Corner, leaving Rarity and Spike alone with each other near the forest.
Spike: Well, Rarity, even though we weren’t able to get the amount of gems you wanted, I had a really good time with you and Pinkie in the gem cave.
Rarity: As did I, Spike. And honestly, just being alongside you and my friends is better than anything else.
Spike: Glad to hear. Well, I’d better head back to the castle and help Twilight with some stuff. See you around, Rarity.
Spike begins to fly off, but after going only a short distance away, he feels something stop him. Looking back, he sees that Rarity is using her magic on him.
Rarity: Actually, Spike, before you go, there’s something I want to tell you.
Rarity levitates Spike over to her until they’re face to face.
Spike: What is it, Rarity?
Rarity suddenly grabs Spike’s face and kisses him on the lips! Spike is shocked at first, but then his eyes turn into hearts and he accepts the kiss. After a moment, Rarity breaks the kiss and whispers to Spike.
Rarity: You will always be my Spikey Wikey.
Rarity lets go of Spike’s face, leaving him in a dazed state. Spike laughs goofily and fly’s off again. Rarity doesn’t stop him this time; she just giggles affectionately at the way he left.

…Probably not one of my better written scenarios, but I’m a little rusty at the moment. ^^;