From Florence to Cinque Terre Day Tour Suggestions for Two

My first recommendation would be DON'T. While possible, a great deal of your time will be spent just getting to and from the CT. If CT is somewhere you really want to visit, then spend a night or 2 in one of the villages. The experience is greatly enhanced by spending uncrowded evenings and mornings there. If you must do it as a day trip, then do it DIY via train. Tours are going to use buses, which will be slower, and you are at the mercy of their schedule.

In September the ferry from La Spezia to the CT should still be running - take it up to Monterosso, then use the train to visit another town or two on the way back to La Spezia.

Cinque Terre is easy to self-guide. Just take the early train there from Florence, walk to each of the five towns via the trail, and then take the late train back. Save yourself a ton of money this way. Cinque Terre is more about seeing and exploring the towns, it's not really a place where you need a tour guide

Wouldn’t call ‘exploring the five towns via the trail’ easy! Lol.

Definitely go on your own by train (or ferry). Rick Steve’s guide has plenty of guidance to help you pick one or two towns to explore. Would 100% recommend staying the night if you can swing it. The CT is an incredibly special experience and well worth the time, $$ and effort!!

Cinque Terre is easy to self-guide. Just take the early train there
from Florence, walk to each of the five towns via the trail,

While I'll agree that the CT is easy enough to do without a guide, "the trail" is not a 'walk'. If "the trail" means the 4 sections of the very popular, often overcrowded Sentiero Azzurro (Blue Path), the section between Corniglia and Manarola has been closed to landslide damage for years now, and the section between Manarola and Riomaggiore now requires a timed-entry reservation (fee). The other two sections involve managing steep terrain and many steps, a park pass, and proper shoes with support and decent tread; they can issue fines now for improper footwear.

There are many other trails in the park system but no need to discuss them if hiking isn't of interest to you?

It would be easy enough to take a train to Monterosso, buy a 1-day treno pass (only needs to be validated first time you use it) and explore the villages by rail. They are all quite small, and only a few minutes apart by regional train + a small bus between Corniglia's train station and the village, which sits above it. You could also do a section by ferry. As far as what to see, the landscape and architecture are pretty much the main attractions; walk about the narrow streets and passageways, stop into the churches, take some photos; done.

Honestly, given the crowds this region is now known for, you'll likely move around more nimbly on your own than with any sort of group. Most of us recommend staying at least one night if at all possible to experience one of the villages during the evening and early morning hours without the day-trippers.

https://www.cinqueterre.eu.com/en/the-five-towns
https://www.parconazionale5terre.it/Eindex.php

Don’t do a day trip to CT.