Two or three nights in Lucerne?

With only two nights, you have only one chance for good weather in the morning for a mountain visit. I would also plan to do a lake cruise. The half day you have after getting there would be enough for the seeing city for me.

I'd say 3 nights, then head to Muerren first thing the following morning. Here is a nice day trip. Take a boat from Luzern to Vitznau, then take the RIgibahn to the top of Mt Rigi hike around, then down, either by cable car from Kaltbad to Weggis and boat back from there, or down the rail line on the backside and train back from Arth-Goldau.

I would definitely stay 3 nights which would give you two full days to explore the city and take the better part of a day to visit Mt. Pilatus. Take a boat on the lake from city center to the cog railway and then take the lifts down the other side. For two kids, the TransportMuseum is a must. Great model train layout as well as actual locomotives and train cars the kids can climb on. Boats, trains and automobiles! Who can ask for anything more?

I vote 3 nights. Lots of good reasons have already been given. The only reason to even consider two would be if it meant you could have another night in Murren.

We could swap it for another night in Murren. 2 in Lucerne and 5 in Murren is completely possible too.

Thanks everyone for your answers so far!

Three nights in Lucerne is great, but if that comes at the cost of reducing your time in Murren, then that’s another matter. I would go for two nights in Lucerne and five in Murren, particularly if you like to walk or hike. Lucerne is very nice, but Murren is heaven on earth.

I agree with Carol. We did 2 nights in Lucerne and I felt it was sufficient. We were there at the end of July and it was very crowded. My last trip to the Berner Oberland we spent 7 nights and I could easily have stayed longer.

If your flight gets significantly delayed, you really lose out if you only have two nights in Lucerne.
And regarding doing Mt Rigi in Lucerne when you are already going to the BO- yes, do it! You have no idea what kind of weather you'll get in the mountains in the BO. If you have a spectacular day in Lucerne, enjoy the majesty of that summit that day. Also, every mountain has its own "personality", is a different altitude, so IMHo, is a unique experience.
Note that with the STP, both Rigi and Stanserhorn are completely covered. That latter has very cool open-air Cabrio cable cars! The kids will love that!
Have a great trip!

Lucerne is lovely. I spent 2 nights there after arriving in Zurich by plane. I spent that partial day exploring the old town, river/lake front, etc. I had planned to spend my full day at Mt. Pilatus, but the weather was cloudy and raining off and on. Instead, I jumped on a train (thank you Swiss Travel Pass) and went to Bern and Murten for the day. I then spent the rest of the evening exploring Lucerne a little more. I know there are a LOT more things that I could have done in Lucerne, but I found my 2 nights there to be adequate. I moved on to the mountains after this stay. In my opinion, I would not sacrifice nights in the mountains in order to add more nights to Lucerne. To me, Zermatt, Wengen (or Murren, if you choose) Pontresina, etc. were much more enjoyable places to spend my time.