Can't go wrong with these:

Beethoven's "Moonlight Sonata"
Bach's "Air on a G String"
John William's "Cavatina" (the one from Deer Hunter)
Tomaso Albinoni's "Adagio" (aka Adagio in Sol Minore) - this one is a favourite at funerals.

Do you have any ideas about what the Grandmother liked? Maybe she had some favourite classical pieces you could choose from; the family would appreciate that, I'm sure.

I hope it goes well for you mate, can't be an easy gig.