Several different sonic screwdrivers are shown sitting in a cup on the Doctor's desk, including models used by the Third, Fourth, and Eleventh Doctors.
The ringtone for Bill's mobile phone is the same as the one for Martha Jones' phone, which was later kept onboard the TARDIS by the Tenth Doctor.
In the Doctor's office there are two busts. One of them is William Shakespeare and the other is Ludwig van Beethoven. Both are characters that the Doctor has met in his travels.
Framed photos of River Song and Susan Foreman (the Doctor's wife and granddaughter, respectively) are shown on The Doctor's desk.
The "star" in Heather's eye is a real condition called central heterochromia, where a person's iris has a ring inside of it which is a different color than the outer edge of the iris. The star shape of Heather's ring, however, is uniquely hers.