We did the pirate theme for my two boys a few years ago using many of the ideas already given here - so much fun! Any kind of scavenger hunt works well with the theme.
Something we did this year for my dd's mermaid party would also be cute with a pirate party - we sent messages in a bottle as invitations. We used water bottles (regular cap/ no sport cap). I added some sand, shells, and green stringy paper (as sea weed),and then we rolled the computer printed invite and tied it with a ribbon (maybe twine for a pirate?) and slipped it in the bottle. People loved getting these, and the really fun part is that you can mail them! We hand delivered the bulk of the invites, but we did mail half a dozen...added about 80 cents in stamps and dropped it in and they arrived just fine (we did weigh first to make certain we had enough postage). For a pirate invite I would crumple the edges and cinge a bit to give it an old look.
Have fun planning your party!