What to pack in summer

We went to Italy for a month prior to Covid in late June and July for 1 month. It was very hot everywhere. We were glad that our hotels had A/C. Some restaurants may have A/C but many do not. I doubt you will want a sweater as even if there is A/C - their idea of cooling is much warmer than we are used to in the US. The restaurants in CT had us eating on the terrace outside and it was very hot but the view was amazing! We went to Italy at the time of year we did so that our kids could go with us, when school is out. I hope to go back to Italy some day but I will pick to go in spring or fall when it is not so hot!

90s F during the day and often a bit muggy too.
At night low 70s.
There are occasional summer storms, so a pocket umbrella is helpful.
Cinque Terre slightly better due to the sea cooling effect.
A light sweater may be needed in the very late evenings, if there was some rain that cooled off the air.

Last time we were in Italy in August, we said never again! It was miserably hot and uncomfortable. Be sure your hotels have AC. CT will be a tad better than Tuscany heat-wise, but hot is hot.

Is likely the high humidity that will feel worse than the heat.
Also to add, some hotel rooms in Italy and elsewhere in Europe have a system where if you take out your key to the room it will turn the AC off in the room.
This saves them money not running the AC while you are out but if really hot outside means you return to a hot room each time taking longer to cool down.

I would bring one light sweater or similar layer just in case you need it for a morning/evening while dining ; even if you never use it would feel better packing 1 in the bag.