2.5 week itinerary to include Cinque Terra, Tuscany and Sardinia..

Mid September shouldn't be cool yet. But you never know nowadays, with the weather getting crazier every year. This past May was like winter.
Not sure about your plans, but Sardinia is an island that is best reached by air (and Pisa airport is in Tuscany and the closest to the Cinque Terre.
September the season in winding down in Sardinia and most flights there are seasonal. Check your options with Meridiana and Ryanair from Pisa.

I have swum in the shallow edge of Monterosso's beachfront in mid-October of several different years, even if it rained the day before, and I'm no polar bear.

I usually visit the Cinque Terre in September, and for the most part it's always been sunny and warm. I definitely had to use the A/C in the hotel. Last fall it was a bit overcast, but still warm.

Been there the last 3 years in Mid to late September and found the weather to be perfect. It was warm enough to wear shorts while hiking the trails, and to go for a dip in the water after the hike too. It rained one day of all the times we were there, and it was a down poor for about 2 hrs, then the sun came back out. We ignored the trail closed signs and hiked the afternoon after the sun was out fo r awhile. Trail was average, some parts were slippery but we all managed.

September weather in coastal Italy is idyllic. Warm, if not hot, and typically sunny. We typically travel to Europe in Sept, and as another reader posts, have enjoyed swimming well into October in the Ligurian Sea. And we are not from Canada! You will be traveling in shoulder season, so there will be far fewer crowds. Wear layers, and enjoy!

Thanks to all replies, very helpful. Are there any comments about Sardinia , is mid -late September a good time to travel there or is end of season?