Italian Riviera from Florence

We have gone to Santa Margherita Ligure several times . Camogli is nearby too.
Portovenere and Lerici are other waterfront towns to check out too. You can take trains.
I do not not know if any of them have places to change your clothes.

Camogli is a wonderful town, easy to get to by train (though it gets fewer train stops so you'll have to change one more time coming from Florence). But it's going to be an extra 45+ minutes each way on the train from Florence vs. just going to the Cinque Terre. If you want to do an easy trip, take the train to Levanto (the town just north of Monterosso, the northern-most village in the Cinque Terre). It has a nice beach and will surely be less crowded than Monterosso (also beaches) or anywhere in the Cinque Terre. Levanto is a nice town itself, even if not the "postcard town" look that the Cinque Terre villages have.

If you are up for hiking, you could take the train to Levanto, check out the town, and hike 2-3 hours south back to Monterosso, then take the train back to Florence from Monterosso. This hike is less crowded than the main hikes between Cinque Terre towns. It's also not quite as nice as some of the other trails but still very worth doing. Still some beautiful views. The payoff comes near the end just above Monterosso, when you get a breathtaking view south all five Cinque Terre villages in one view. The hike from Levanto to Monterosso is also free (other trails require a paid pass).

You could also train hop to another village before heading back to Florence, some villages have fewer train stops. More trains stop in Levanto and Monterosso than in Vernazza, Corniglia, or Manarola so easier to get to/from Florence from Levanto and Monterosso. Check train schedules. It will definitely be crowded inn any of those towns mid-day but by late afternoon maybe thinning out. Depends how late your train back to Florence would leave whether it's worth doing more.

There's also boat service between the five villages (except Corniglia) and the town of Portovere nearby. The boats will be mobbed and full in June, but if you can put up with that, you could take the bus (I think) from Portovere a short distance back to La Spezia train station and get back to Florence from there. Portovere is probably going to be a little busy too but not as busy as Monterosso, Vernazza, etc. I'd guess.

Start with the train schedule out of Florence and back in the evening - decide how much time you want to devote to the trip. And work out what you want to do in between. Maybe wing it a little when you get there, if you know when your train back to Florence will leave. Just give yourself plenty of time at the final train station to buy tickets etc. There are ticket machines and human ticket sellers at some stations or phone apps if you have smart phones. (If you are taking regional trains instead of IC trains, regional trains can't sell out - no reservations, no discount for advanced purchase). But, doing Levanto, hike to Monterosso, train from Monterosso back to Florence should be very doable, maybe with room for another town or two in between.

Given the short time available (the trip from Florence is at least 2.5 hours each way) and the fact you are interested in beach time, below are the best options along the rail line and with a real beach (Liguria is very cliffy and rocky, so beaches tend to be few and pebbly). I'm listing the from East to West, in order of distance from Florence.

Monterosso - (2.5 hours with at least one train change)
This is the only Cinque Terre village with anything that resembles a beach. The sand is fine pebbles, but it's large enough and not total rocks like the other towns. Unless you go on a weekend, in June the beach shouldn't be overly crowded.

Levanto or Bonassola -
These two towns, respectively, one and two rail stops after Monterosso (5 minutes away). The benefit of both is that they are very close to the Cinque Terre

Moneglia
My favorite town when I was a youngster. Nice beach and tacked away from the rest of the world. It's within 3-3.5 hours from Florence. It's a small village so only Regional trains stop here (not InterCity trains)

Sestri Levante
The next town after Moneglia, a bit further west in the direction of Genova, but still under 3 hours from Florence, since it is a major train stop so faster trains (Intercity) stop here (unlike Moneglia where only Regional trains stop).

I'm not going to mention places farther west, like Rapallo, Santa Margherita (Portofino) or Camogli. They are fine choices but since it's a day trip you are starting to get further away from Florence. Porto Venere and Lerici, mentioned above, are the closest, but with no rail service, therefore you would need to hop on a bus from La Spezia.

Thanks so much for this information. I saw some mention of Castiglioncello in other posts. Anyone aware of this is some place and have thoughts?

I've always liked Castiglioncello, and so did many of my friends in Florence. It's pretty and it has similarities with the Ligurian towns, but much closer to Florence. It's just minutes south of Livorno, which has very frequent service from Florence. I think some trains that go from Florence to Grosseto, stop also in Castiglioncello and don't even require you to change trains in Livorno.

Thanks Roberto. You are a wealth of knowledge! Anything else we MUST do while in florence? We have been before (loved it), but first time with our 17 yo daughter and nieces, so will show them the sights, but also want to do somethings that are special and not make it exactly like our first trip. We are going to go to Siena too for a day trip. any tour group or guide that you might recommend? Thanks so much!