Day Trip from Florence in Early July: Alternatives to Cinque Terre?

July is hot everywhere. In order to get some fresh air you would need high elevation and you won't find it in Tuscany (well, maybe Abetone pass). Hill towns may be somewhat fresher than large towns in the plains, but only during the night.
Among the places you write, val d'Orcia or anything south of Siena would need a car to be feasible as a day trip. If you are on public transportation I would consider Lucca or Siena.

I would go with Siena. It’s about an hour away from Florence. It’s a beautiful medieval town, you won’t find better attractions than the duomo, the piazza, and just walking around town. If you like art, the duomo is as good as it gets with masterpieces all around.

Yeah, another vote for Lucca, perhaps combined with Montecatini Alto. Much of the walk along Lucca's walled fortifications is shaded, and you can also rent bicycles to ride along the walls or explore inside the town.

Hi,
I've been to all of the places you mentioned (except San Gimignano) at least once, and I agree that it's going to be hot no matter where you go in July. I loved all of them and I suspect you will, too. If you are without a car, then Val d'Orca is out (unless you take the bus to Montalcino or Montepulciano - which I've done). I'd choose Siena as it will provide you the medieval town, scenic landscapes, especially if you climb to the top of the unfinished Duomo, and lots of cultural gems. I just did a trip from Florence to Siena (second time) in November and it was a piece of cake - quick easy train ride about 1.5 hours each way and it wasn't crowded at all. I'm told that the bus from Florence to Siena is more efficient as it drops you in downtown, whereas the train station is just outside and well below the walls (there's lots of escalators in case you don't want to take the stairs). However, as others have noted, you can't go wrong with Lucca or Pisa.
Good luck

If you must absolutely go to the breezier coast and you are willing to wake up early, and take a taxi to the Firenze Campo di Marte station (just outside the city center near the stadium), you can take a fast FrecciaBianca train at 7:54am which will get you to La Spezia in less than 2 hours. Then from La Spezia you can take one of the many regional trains to the Cinque Terre. The same exact train back to Florence departs from La Spezia centrale at 7:27pm. But of course there are many more slower options.

Another option would be to go to Castiglioncello, a pretty coastal town near Livorno which is closer than the Cinque Terre. Most trains would require a change in Pisa, but you could make it there in just over 2 hours.

The towns in the interior of Tuscany will be scorching hot everywhere.

Lucca.
Up on the walls is shaded by trees.
It’s a delightful small city.