1) since the hole is already there, let's hide some of it in a pirate ship! it would be a fun 'below deck' area!
2) or let's make a trapdoor in the deck of the ship to a hidden sandpit (it's like a buried treasure hunt!)
3) go big or go home! why build just an 'impression' of a ship, when really kids want doors and ladders and slides and planks to jump off of?
So dad....when do we start?