Cinque Terre, no hiking, still worth to go?

Margaret, YES, the Cinque Terre is absolutely worth a visit, even if you're not hiking. While hiking the trails seems to be a "ritual" with many travellers, that's not essential. You can still visit each of the towns by train or boat. Driving to La Spezia would be easiest, as there's a large car park near the station. From there, take the local trains to each of the villages. Tickets are cheap, but don't forget to validate! The C.T. is very different from the towns on the Amalfi Coast, so that will be a different travel experience. Be sure to try some of the local Pesto in the area where it was invented. If you want to try Pesto Lasagna or Trofie al Pesto, I can suggest some restaurants in Monterosso (I was there about a week ago). Happy travels!

Yes, you can still enjoy the CT if you don't hike. Trains connect all the villages as well as boats.

Absolutely. Half the time the trails are closed off for one reason or another anyway. Best way is to drive or take the train. You can use the ferries to get between the different villages. Beautiful from the water. Not all year do the ferries run.
Check the schedules.

Although there are still "things" to do in the CT, I personally do not feel that it is worthwhile to go that much out of your way based on what you have shared of your itinerary. The highlight of our trip to CT was the hiking, and also just lounging around on our balcony on the cliff with wine and great food. The latter will also not be an option for you since you appear to be doing a day trip. The worst thing about the CT was taking the super crowded and hot trains (the boats were also hot and crowded).
Since you will be visiting the Amalfi Coast, I suggest hanging out more in Tuscany.

Do the train and boat/ferry has air-conditioning?

Do the train and boat/ferry has air-conditioning?

You will not need air conditioning on the Ferry the sea air will be sufficient. As for the trains - from Siena to La Spezia if you go 1st class maybe, then you will be travelling by regional trains and having taken them I did not experience any air conditioning from town to town. Again if you are only doing a day trip I would not do it and concentrate on Tuscany. Make it a point on your next trip to spend 3-4 days in this region and see a bit of all the towns and Portofino as well. Spectacular area. Of the 2 (Amalfi & Cinque Terre)Cinque Terre especially Vernazza gets my vote. In mid May the crowds were very light and not as busy as the Amalfi.
Whatever you decide enjoy.

There really is no air conditioning on the regional trains, which is what you would be taking between the villages. It is certainly manageable, but if you have no where to retreat to after the ride, I can see it being unpleasant.
I concur that if you can get back to this region for a few nights, do it that way. Otherwise, spend that day in Tuscany. Maybe visit a working farm/winery, with a little easy hiking thrown in.

Agree that it is manageable to have a day trip to CT but it will be quite rush. 3 hours one way from Siena to CT and total 6 hours on train may not be seen as justifiable. Many thanks.

Agree that it is manageable to have a day trip to CT but it will be quite rush. 3 hours one way from Siena to CT and total 6 hours on train may not be seen as justifiable. Many thanks.