Hi Naumann -
Whether it is boring or not depends on your own perspectives. I have traveled most of those routes at one time or another, and they are certainly scenic.
Please look at the map that I linked you to earlier.
You will see that the three trips on Lake lucerne cover different branches of the lake.
I would suggest the trip to Flüelen, because I think the southern part of the lake (called the Urner See...."See" in German can mean "lake") is particularly dramatic. I posted a few pictures from that region in this link about hiking in the area:
At the end of that trip (at Flüelen) I'd take a train back to Luzern. Or, if i were to repeat part of the boat trip on the return, I'd ride from Füelen to Brunnen, get off there, and take the train from Brunnen back to Luzern. The train station is a 10-15 minute walk from the boat landing, but there is a bus that runs three times per hour and takes 11 minutes. Your senior member of your party would not have to walk.
And, if they really like the ride, you could stay on the boat and return from Flüelen all the way to Luzern that way
Check the boat schedules...some make more stops than others. Watch out for the different seasons listed on this link.
if you do that, check the train schedules in the time table to make sure you have a good connection.
The other two rides duplicate the first part of that trip, but then go to different branches of the lake. Alpnachstad is the base station for the cograil to Pilatus Kulm.
I'd only do one on each lake unless you have a lot of time on your trip. That also is a personal judgement based on how I allocate my own time. Your preferences may differ.
Also, on Lake Geneva, I would also study the map. Those trips are very different, but the lake does not have branches the way Lake Lucerne does, so you would see somewhat similar scenery on all trips.. I'd also consider using a train for the return on the Geneva trip, but I'd go in the reverse direction. That is - boat from Montreux to Lausanne/Ouchy to Geneva. Or, take a train to Geneva and return by boat.
The train station at each stop has varying distances from the lake boat piers.. I don't know the bus link details in Geneva, but the train station ( Cornavin) is 10-15 minutes walk from the lake. At Lausanne, there is a nice funicular from Ouchy direct to the main train station. The lake front at Ouchy offers many nice restaurants.
On Lake Geneva, a particularly nice stop is in Yvoire: