Cruise Ship tours for cruise lines just started last year, so depending on the cruise line’s time in port will determine whether you get the bus shopping time in North Conway on the way down from Crawford Notch. So yes, it is a one - way train ride from North Conway to Crawford Notch and if the ship is in for 10 hours or more, you then take a bus from Crawford Notch to North Conway for an hour “on your own” to visit the town. If your ship is in for shorter period of time, the time in North Conway will probably be limited or eliminated all together simply due to timing of the cruise ships time in port. Either way, it is the greatest way to see fall foliage and it’s spectacular in the summer also. Box lunches are onboard the train and I think there was a snack on the bus ride from Portland to the train.
I’m not sure there is any “filling of time” as you’re actually are limited by the time that the ship is in port for this full day tour.