I'm walking down the aisle to Tiny Dancer by Elton John! We both love the song, it has some happy memories for us and the tinkly piano bit at the start will be perfect.
That said, my cousin got married in a church a few weeks ago (we're Register Office) and they walked in to an amazing gospel choir that even impressed us non church goers. My mum proceeded to pump the bride for info in the receiving line, even though we live hundreds of miles away!!