How many people? How much luggage?
I looked at daytrip.com and there is no easy way to specify what you want, since it is somewhat of a canned website quote engine. They do allow stopovers, but only short term and along the direct route. Since Luzern is not on a direct route between Basel and Zurich airport, does not seem doable. But to give you an idea, 3 people with extra luggage is about $345 from Basel to Luzern, and another $345 from Luzern to Zurich airport. Having a driver and car hang around for 5 to 6 hours is bound to be another $500, so I think my guess of 1000 CHF is not too far off. Are you good with that amount of money?
How old is "older"? Do they have mobility issues? How do you explore Luzern with mobility issues? I would just say that "older" being under 80 with good mobility, the train is easy peasy. An XXL locker at Luzern station costs 12 CHF for 6 hours and is 20"X 33"X 37", so it holds a lot of luggage.