I assume you're looking for serious replies so I won't suggest "Your Body is a Wonderland". 😀
At ours, my college pal was playing the organ. He's awesome so he improvised something appropriate to the occaision of the bride-as-vision-of-beauty-walking-down-the-aisle. I assume your harp player is not an improvisation expert (organists must be as part of their jobs, after all). Please don't use the "Pachelbel Canon". As much as I like it, it is waaay too "done", much as is "Here Comes the Bride". I suppose your choice will depend on what the rest of the music is like.