A.W.
Anything "secret" is always cool! A spying scope portal, a hidden cubby, treasure area, telephone. Something with a simple pully system to raise and lower something ( our fort had a plastic trash can lid that we could raise up to it once we had climbed up filled with toys/whatever). Anything old with knobs and dials and buttons - an old radio, CB radio etc. You could make a mini "command center". maybe a place on the wall to hang his pirate sword? On the porch you could put a ship steering wheel so he can pretend it's his boat and he's captain!