But let's throw away the nuclear option straight away: if you even want to make it on to the flight to Europe, any suggestion that this request is ridiculous/unreasonable/irrational is a Very. Bad. Idea.
You could try to ignore it, hope it is not mentioned again and that it is, ideally, forgotten. This is possible: your trip is several weeks away, you're both busy people with a lot on your minds. You ought to be able to pick up a nice souvenir at the airport before your flight home. A nice scarf, perhaps. But do you really want to risk this option and the potential consequences? How likely is this based, on past experience?
I thought so.
Maybe you could convince them that online shopping is not just the future, it's the present too. And there's no time like the present to shop for a timepiece. Ha ha. If dodgy wordplay fails, pointing out that online you have a bigger choice might swing it. Explain that you can relax on the couch at home, looking at the pictures and videos, comparing prices, before finding just the perfect clock.
It's a plan, but it's unlikely to scratch the itch.
If you've done your research, you might suggest a drive to a shop in a town a couple of hours away that sells fair dinkum German cuckoo clocks. Just like they sell in Germany. The shopkeeper looks nice on the website so you're guaranteed to get friendly Kiwi service. Let's take a leisurely detour and have lunch at that wunderbar German bakery in Waihi. It'll be fun. We can make a day of it.
But, come on, you just know that Kāingaroa Forest does not equal Black Forest.
Finally, you could appeal to their altruistic side: wouldn't you want me to have a hassle-free trip? There are all those logistical difficulties: finding a shop, negotiating a good price, having it packaged up, worrying about whether the precious memento will make it safely back to New Zealand. Would you really wish all that on me?
The silence will be deafening.
Cuckoo! Time's up! The correct answer, liebling, is: none of the above, because none of it deals with the fundamental issue, which is: when your other/mother/lover can't be there, only a direct physical connection to the Dream Destination, through you, will make up for it. And there's nothing cuckoo about that. Buying an actual clock, in actual Germany, and actually carrying it home to place into the hands of your darling is a sign of love. And that, actually, is timeless.