Me and my family had actually talked about this a number of years ago.
My parents said mine should be "I'll be back."
Dad's was - "Beneath this sod lies another" - in case someone doesn't know, sod is also the name for a lump of earth.
Mum's was - "Well she was tired."
One of my older brother's (who's not really a nice bloke to be honest) said that his would be - "Wish you were here."