Hi Florat,
That's nice that your parents can come and visit. I'm sure it will be a special trip for them, whatever you plan
I agree with keeping it simple - a week is a very short time and you'll want to maximize value for time/money, minimize stress and of course enjoy the time with your parents.
I agree with all of the Luzern fans. It's a beautiful little city and you can experience so much from there.
Without nailing the itinery down, I would plan to include:
the Geneva -> Lausanne boat tour with shopping for the French feeling.
Luzern - old city tour shopping and the city wall/clock tower (check open times!! It's closed in Winter) and boat ride.
+ either the pilatus golden round tour on the boat, cogwheel train and gondola
or
a trip by train then gondola up Titlis (as suggested above). Titlis is higher (3000m above sea) compared to Pilatus (2000m above sea) so the snow and views up Titlis would be better. (Note that it will be colder up there so get your parents to take a warm jacket). Engelberg, the town near titlis is also a beautiful old town with character.
From what I've heard you have to strike it very lucky to get good conditions up Jungfrau and Zermatt. And it's REALLY expensive.
I would avoid Zürich except for a wander down Bahnhof strasse on the way to/from the airport. It's a pretty enough city and Uetliberg is nice, but you'll get plenty of views in Luzern from the clock tower or the mountains.
The rest I would leave open for sponteneity to either chill or decide at the time to stay at one place longer etc.
I hope you and your folks enjoy the trip either way!
Puddy