Any of the songs in any of the hymnals are written such that you can comtemporize and personalize them. Pick one of your favorites, or, if you know the theme of the service, pick a appropriate song that matches the theme and "embellish" it. Don't totally hide the melody. Keep enough of the melody that folks can recognize it and follow along with the words in their heads if they so desire. Plenty of opportunity to improvise in those tunes if that's what you're looking for. If you've got something to say that will add to and not detract from the intent of the original, that is.
Good luck!