This map could be useful.
In the menus, under Points of Interest/Traffic, you can turn on or off the icons showing all the transport stops, including mountain railways, lake boats, etc.
The drive around the Thunersee (Lake Thun) as mentioned by Annika and by Arno would be very nice, although the driver would have to spend more time looking at the road than the scenery. The train and bus is a good option. Frequent service allows you to get on an off at, for example, Thun or Spiez and catch a later connection.
I have attached a screen grab from the timetable for a few hours in December 22nd. It shows trains twice perhour taking 34 minutes from Thun to Interlaken Ost (IO) and a bus once per hour via Oberhofen and Merligen. By clicking on the "+" to the left of an individual journey, you can learn about intermediate stops and a map can be selected from the footnotes.
if you have not used it, you will find it quite helpful for planning your rail trips, bus trips, etc.
That said, Annika's recommendation -
<<"If weather is clear on December 22, I'd strongly recommend to do some sightseeing in the Jungfrau region rather than driving around Lake Thun. You'll find beautiful snow-capped mountains there.">>
comes from an expert who knows the region intimately.
It is a good suggestion. Please check the link that she provided.
A trip to Männlichen via cograil from Lauterbrunnen to Wengen, followed by the large-cabin cableway up to Männlichen for lunch would provide spectacular views from a high altitude perspective of a region with that is much larger than Zermatt. it is famous for good reason.
I have attached a few pictures taken in warmer weather, to give some sense of the vistas.
You could leave your car at Interlaken Ost or Wilderswil to provide a base for a circular round trip returning via cable way from Männlichen to Grindelwald. Or, you could ride the cograil all the way up to Kleine Scheidegg and down to Grindelwald. At Kleine Scheidegg, you could choose the very expensive branch train line up to Jungfraujoch, but it is not necessary to do that to experience the scope of the region.There are other options as well.
There is a large car park/ garage at Lauterbrunnen. It has a a lot of capacity, but i don't know how full iti is on December 22nd. It makes a circular route a bit more awkward, but could be effective for a trip up to Männlichen and back via Wengen.
Drive (or train) from Bern to Luzern:
The scenic drive through the Emmental via Langnau im Rmmental and Schüpfheim is very nice. it is not fast. The Emmental is a characteristically Swiss farming region with rolling hills and raoil and roads through the valleys.
It is also good on the train. The timetable defaults to the fast route via Olten or Zofingen, which covers a longer distance faster because of new stretches of fast track. Trains which avoid Olten are faster.
Trains via Langnau offer good views of the Emmental.
Here is the website for parking in Luzern:
There is a parking cost calculator.
P1 by the Hauptbahnhof (main station) is often full.
As you drive into the city or near the Bahnhof, there will be signs showing which parking areas are full, or how many spaces remain free.
I find driving in cities such a Luzern and Zürich to be difficult, even though I routinely drive in New York City, and in the USA's driving equivalent- Boston. The streets are at best only partly on a grid, and one-way traffic is common. Turns come up quickly, and the road signs take a moment to register, since you have to deal with the German names.
An option to consider if you do not wish to drive into Luzern is to park at an outlying rail station, and take the train into the city. It would take some time out of your day, of course.
For example, trains from Malters to Luzern run twice per hour and take about 12 minutes.
On the line from the northwest, there are three trains per hour from Sempach Station ( distant from Semach town) which take 20 minutes.
I don't know what the parking patterns are at these stations in the days leading up to Christmas. in non-holiday times, i'd expect to find parking there.
here are good reasons to do that part of the trip by train. One argument against it is the need to haul luggage on and off the trains. Lockers are available at most stations. larger ones have "left luggage' offices.