This was my fantasy Christmas from when I was old enough to get the concept of having a dog! Waking up on Christmas morning to find 'clues' and eventually a puppy. Lucky, lucky girl. (Yes, I know the puppy wont be there, but I'd have understood that and spent the entire month like this 😬😬😬😬😬)
There's a big difference between getting a puppy when the time is right & you've thought it through and just 'getting the kid a puppy for a present cos the kid will think it's cute' it's lovely to be able to tell her on Christmas Day,
If people can't understand that, that's their problem.
She's 12, so I'd go with 'not too obvious' & building up, something like...
A calendar
A note to open the Calendar & spot something unusual (date circled)
A rolled up newspaper (that might be too dated now)
A new pair of outdoor boots/shoes (if she needs them)
A storage box (whatever you'll put the digs toys in)
Anything else that will be buying anyway, that will make her go 
Then a photo frame with a picture of your puppy and a voucher for Pets at Home as I'm sure she'll get more fun out of choosing the bowls, lead etc herself.
I'd love to be at your house on Christmas morning 