A pirate party theme is great for creative invitation ideas
You could create some mini treasure map scrolls
Use thick paper, watercolour paper is ideal. To age the paper start by tearing the edges , then to stain use either tea or coffee mixed in boiling water, you can use watered down paint but tea and coffee give a great aged look and the patchiness of the colour adds to the effect. If you want to emphasise the patchiness you can leave an old tea bag on the paper after you have removed it to dry.
Once you’re happy with the colour (you may need to leave it in the mixture a while to penetrate the paper), leave the paper to dry completely
But if you’re not the most crafty or just don’t have much time below are a few free printable invitations.