A few years ago I was at Disneyland with my then girlfriend where we witnessed the details of this story...
As we walked through New Orleans square, we noticed a few people looking a character sketcher drawing a young couple which is always fun to watch and the crowd seemed pretty engaged... so we stopped.
At the top of the artist's desk was a sign saying "Shhh... Don't Ruin the Surprise". Of course this was facing our way while the couple just saw a blank piece of paper.
The artist's however had ditched the casual clothing and seated stance of the couple and instead was busy drawing them in traditional wedding attire: the guy on one knee, the ring half way on the finger of the girl and the caption "will you marry me?"
It's not unique for people to stop to watch a sketch in progress but watching the whole thing or a huge group would have certainly been strange. But people were quite clearly on their best behavior that day walking off as we walked up and leaving just a small audience.
Our timing turned out to be pretty great as we arrived just in time for the ending. The artist turned the sketch around, the guy dropped on a knee to match, ring in hand to complete the pose, and the girl [who was quite clearly not expecting this to be the moment] gave an enthusiastic yes while everyone watching cheered.
My only regret was not scoping out the scene better. The couple was apparently alone and while my dSLR was already out, I didn't think to use it. Those photos would have been something else.
Simple, and, as far as I know, completely original. That guy gets major points in my book.