When booking train travel remember if doing long journeys to book at least 90 days in advance to get the cheaper fares called supersavet fares.
Stresa & Baveno
Stresa is the more popular of these two destinations but Baveno appears a little more chilled and has a better view of the Borromeo Islands.
You can take the Maggiore Express from Stresa to Dommosola, then on to Locarno via the small train. Once here you have 1-2.5 hours in the town before boarding the ferry back to Stresa.
You can also do the trip in reverse but have less time in Locarno by doing the boat first.
Either way it costs 34 euros for the day trip which is great value. You can add on another day to explore the island better for an extra 10 euros which is cheaper than booking them separately.