Play as Grandpa
To unlock Grandpa as a playable character, using Grannie, go to the level Ghost 4-4 (top right corner level in the Ghost stages). Once the level starts, climb the first set of pillars and super jump straight up. There is a "prison" there that you need the power shot to open. Once you beat the stage, you can then quit to the file select screen and press L to cycle through to Grandpa.