The signatures of HM, Charles and Edward etched on a window:
There are also a couple of other signatures which can be seen in the photo, but I can't work out whose they are.
The 2012 Christmas Card of the SRF features Felipe and Letizia's signatures- Leonor and Sofia have also signed their names at the bottom:
Leonor has very good writing for her age IMO. When I was 7, I was just learning how to write "joined up". The way she writes her r's is very nice- I wish I could write mine like that!
Queen Maxima, King Willem-Alexander and Princess Beatrix's signatures: