River carries a sonic trowel. While this reflects her profession as an archaeologist, it is also something of a family tradition: her grandfather, Brian Williams (the father of companion Rory Williams), always carried a trowel.
Steven Moffat intended for a while to have this be his last episode before passing the show to a new runner, hence his using it to finally tie off his very first original plot thread. Then he decided at the last minute to stay a while longer.
Among the items River Song removes from her bag to make room for the head is a red fez.
This is the first episode where both the Doctor and his companion are played by people over the age of 50.
The Doctor says that River was once married to Stephen Fry. Fry has a long history with the show, going back to before its revival. He provided the voice of a character in the animated mini-series Doctor Who: Death Comes to Time (2001) and once wrote an episode during the Russell T. Davies era but it required rewrites to make the budget more manageable and he couldn't do them in time (the slot was taken by Fear Her (2006), which, at the time of this episode's broadcast, was the joint lowest-rated episode of the new series).