How sweet! I'm sure they'll love whatever you do, but I think a book would be better than a calendar. That way they can store it on a bookshelf and pull it out any time they want. Once a calendar's year is up, the only thing they can really do with it is put it in a drawer, unless they want to leave an outdated calendar hanging up.
As for how to do it, some of the pages can be text only, but I think you'll need at least some photos. I would start by writing the text, then look for photos that will help you illustrate what you've written. (Just make sure they're high-resolution photos or they won't print well.) And if you have a decent camera, take a few of yourself. I'm sure they would love to see you among the photos.
Good luck! And post at least a few of the pages after you're done.