The villages are so small that I'd do at least 3 of them, not including Monterosso as it looks like you'll be basing in-or-near-that one so you can cover it the day you arrive. You could take a ferry all the way to Riomaggiore, explore a bit, and then work your way back; walk the Via dell'Amore to Manarola (you'll need a timed-entry reservation for this):; train to Vernazza; train back to Monterosso.
https://www.parconazionale5terre.it/Eiti_dettaglio.php?id_iti=3575
https://www.cinqueterre.eu.com/en/cinque-terre-via-dell-amore
You could also, weather permitting, hike the "Blue trail" to Vernazza, continue your hike to Corniglia; train to Riomaggiore or Manarola: ferry or train back to Monterosso. Corniglia is tiny but was a favorite. I'll caution that the "Blue" involves lots of steps, can be VERY busy, and requires a park Trekking or Treno pass. If using the trains more than twice, I'd get the 1-day Treno pass.
There are some high spots/view points in several of the villages that can give you a bit of a workout without tackling any of the trails at all, such the Church of San Francesco/Monastery of Capuchin Friars in Monterosso +nearby cemetery, and cemetery in Vernazza. As well, some of the villages involve number of steps + inclines to get around.
https://www.cinqueterre.eu.com/en/
https://www.cinqueterre.eu.com/en/cinque-terre-card
https://www.parconazionale5terre.it/Eindex.php