Do you have any plans for beaches or hiking while on land? Just the towns themselves? Meals besides lunch?
On two trips, we stayed in Riomaggiore both times, and it’s my favorite of the five - although the enormous crowds in the general area in September 2022 have made me doubt I’ll ever go back. I suggest you go with Plan B - go directly to Monterosso, then afterwards, head south from there. Save Riomaggiore (the best) for last.
Vernazza is Rick Steves’ favorite, but the main street was packed literally shoulder-to-shoulder with pedestrians on our last visit. You’d likely center around lunchtime, and would have some dining options, unless you stick around bigger Monterosso.
Tiny Corniglia is actually worthwhile, especially for the Alberto Gelateria gelato shop, but it’s inland, and requires climbing LOTS of steps above the station, so with just a day for the region, could be skippable. There’s a shuttle between the station and the town, but catching it, and not finding it completely full, was hit-and-miss.
Manarola has a bit of hiking through vineyards available from right in town - free, and no pass required. Not super-long if you’re not looking for a big trek, but they’re steep, and definitely harder than a simple stroll in the park.
Riomaggiore has some hikes, too, and a secluded beach with stones, not sand. Several good dining options, but if you’re having dinner on your ship, or need to be back on it by 8:00 pm, you’d need to plan accordingly.