Mt Pilatus, just outside Luzern, is a good mountain to go up if you don't want to do lots of hiking. I recommend the Golden Round Trip, where you can take a boat ride on the lake, take the cog railway up the mountain, enjoy the view from the restaurant, then take the cable-car down back to the city.
Here is the link for the round-trip (also nice because you just buy one ticket for all the transport):
http://www.pilatus.ch/en/railway-cab...-trip-195.html
Also, I know you say that city sightseeing isn't their thing, but Luzern is really very pretty - especially because of all the intricately painted building facades - and might just change their minds!