If Pinkie Pie were doing this, it would just be Pinkie doing something typically fun and silly for one of her friends. But with Rarity being so unlikely to do something this unladylike, the context is much sweeter!
Like HalfDeathShadow suggested, I kinda hope they'll switch roles once they're in private...with a few changes: Spike's harness will come with a gag and whip, and the sled - decorated with the female symbol - will be much harder to pull.
Adorable. Spike looks so happy there, and Rarity? Leave it to the Element of Generosity to play along so that a friend can be happy. Generosity takes many forms, and not all kinds of giving involve physical goods.