A great though longish day trip to take from Lugano goes via the Palm Express bus ride thru the terribly scenic boulder-strewn Majola Pass to St Moritz, where you board one of Europe's most scenic trains (if not the most scenic in my opinion) - the Bernina Pass trains that take you up over the Bernina Pass (the highest rail crossing of the Alps - a land of glaciers, alpine lakes and totally awesome scenery) to Tirano, Italy where you hop another bus to return to Lugano - you on trains or buses most of the day so it's not for everyone - a better way would be to go from Zurich to St. Moritz, stay there and then do the Berinina train the same day you get there (going just an hour or so to the most scenic part and back to St. Moritz) and the next day take the Majola Pass bus route to Lugano. (All buses and trains valid with a Swiss Pass; buses require a reservation.)
this way you'd knock a day off Lugano but have the unforgettable memory of the extraordinary Bernina Pass train route! 2 nights is enough for most in Lugano.