From your posts I'm guessing it's a birthday party for a small child so I recommend some mlp themed treats like mlp cupcakes with cutiemarks, could try to make donutopia but that might be too time-consuming but you can still make other treats from the show that humans eat most of them will be apple based though so I recommend also making some non apple treats for those who don't like apples.
You can also have some pony related games (pin the tail on the pony and whatnot) and decorations (pony balloons, banners and so on) and if you want you can have some music but I recommend to have some music from the show rather than fan-made music if you want mlp music but you should just make sure the music is happy.