Cut the top off of the pineapple with a sharp knife.
Scoop the inner flesh out of the pineapple using a serrated knife and an ice cream scoop or melon baller. Set aside the scooped fruit for later use. Leave about half an inch of the fruit intact to keep the wall of the pineapple sturdy.
Place a glass of water inside the cored pineapple. Don't fill the glass all the way to the top to avoid spillage, in case the bottom of the pineapple isn't completely level.
Stick your flowers inside of the glass of water so it looks like the pineapple is the actual vase.