Thank you for all your helpful suggestions. One of them - a grandchild - didn't get passed on to the Lovely Son and girlfriend though, for obvious reasons (well, timescale and expense, anyway, as well as it being None Of My Business, delightful daydream though it was).
No, the Christmas present suggestion came to me in a blinding flash.....
I looked out yearningly at the sodden mess of mud and bleak paving slabs - currently littered with builders' buckets and ladders - that I hope will one day become a lovely little garden, and thought I would like an apple tree. And perhaps a plum. Or a pear. Or a cherry. Or a .... oh, I don't know. Definitely an apple tree for starters, anyway. After all, we are in Somerset now.
So that's what I'm being given for Christmas.
(whisper: although a grandchild would have been nice too......)