Definition of Swiss Travel Pass Flex “travel day”

Can you please clarify the definition of a travel day when on a Swiss Flex Pass? Specifically, if we spend the day visiting castles in Bellinzona and never get on any sort of train or bus, just walk to the castles, can we call that a travel day? I will definitely have the pass but only need it for two full days of travel (with kids so the math works out for us!) so wanted to use the third day for castles/museums only. Just wanted to make sure it’s not required that I get on any sort of public transportation (bus, metro, train). Unless perhaps it covers the tourist train that takes you around the castles in Bellinzona? that’s probably a long shot.