Though I am normally the first to say, “embrace the place and stay an extra day,“ half of the second day of the itinerary for Cinque Terre is actually a half day trip.
You can buy the Cinque Terre Card that includes unlimited train rides (and bus rides, which is an alternative to the train) and also some other perks (like using the toilet at the train stations for free and a hiking pass).
Riomaggiore (“Lover’s lane”) – 30 minutes (1.1 km), this easy Cinque Terre walk is closed for longer
If needed, you could hike all of the trails in one day, but if you have the time, I would recommend splitting them up so that you have time to explore the towns between trails.
If you plan on both hiking AND taking the train, purchase the Cinque Terre Train Card, which is the same as above except that it also allows for unlimited train travel within Cinque Terre.