I made a Pirate Ship cake. Actually, I constructed it from those marble-cake loaves, stuck together with butter icing. Hull, a chunk for the fore-deck and another at the back. I covered the hull in choc fingers to look like planks.
I have also made two Treasure Island cakes. One was an island, made of brojen up sandwich cakes, with a small mountain in the middle, a cove etc, all put on a board covered in sea icing, with the island iced in green mountains, yellow beaches, a blue waterfall..all done with coloured roll-out icing.
the other was a treasure map cake - much easier. Cake covered in 'parchment' colour roll-out, with directions and features done in the writing icing. Treasure chest with foxes galier fruit jewels etc.
Also, pirate buns. fairy cakes decorated as pirate faces. White basic icing, eye patches from black roll-put cut into rounds with a pastry cutter and cut in half, hair, mouths, iced on with writing tube icing.
Selection of pirate-theme games?
Play 'what's the time captain hook?' instead of 'Mr Wolf'. Have a treasure hunt with little barrels wrapped in gold foil from the pick'n'mix - or gold coins, play musical statues, but they have to make statues of characters from Peter pan, do Pass the Parcel, to a selection of piratey sea-shanties (DP downloaded from iTunes), and as it was hot, we did walking the plank fom the garden table into the paddling pool - but that was considered downright irresponsible by some of the parents 
(or maybe, as it was a P pan party, it was the 4 year-olds brandishing a broom handle to smash the croc pinata from Woolies that they thought was dangerous!)
have fun!