There’s the information in my examination of “The Eleventh Hour”, including the fact that Smith’s first episode contains a hint of what eventually happens to the Ponds. In that post, I go on at length about the strangeness of the swing.
In “The Power of Three”, the Doctor starts to take his leave at 39:00 by saying, “Worlds to save, swings to… swing on.”
I hate when my brain does this. I think I’ve just spoiled the next episode for myself completely. Don’t read further if there’s anything at all you like about surprises, in case I’m right.