I came across this novel approach to a wedding RSVP, which was originally posted to the Reddit site.
The brutal honesty of it is probably why it's gone viral on the Internet, but I was most excited about the last part: "Please list a song that will get you dancing!"
Anyone who has had a wedding or been the DJ for one (I've done both!) knows it can be a challenge to find the right song. With this invite, you can crowd-source the playlist.
Of course, you're depending on people actually being honest. Otherwise you might spend the night listening to Leonard Cohen instead of Kelis.