What is inside the little house on stilts in celestic town?

There is no little house on stilts in Celestic Town in the Pokémon games. Celestic Town is the town where the player starts their journey in Pokémon Diamond, Pearl, and Platinum, and it does not feature any houses on stilts.

It's possible you are thinking of a different location in the game or a different game entirely. Could you please provide more details?