18 Days first trip to Italy - Looking for thoughts on my planned itinerary

You're moving around more than I would want too, but its probably doable.

It is doable, but you have a lot of one-nighters. You do waste a lot of time packing up, checking out of hotel, getting to train and next stop, and checking into hotel again. All are lovely cities, though!

I would not overnight in San Gimignano (close to Siena) and Lucca (close to Florence) for one night each. Too much hassle for me, changing hotels. Neither one of those overnights will save you much, if any, transportation time as you head to your next stop, unless you will have a car.

How do you plan to move around?

We were in Siena last year six nights and loved it. Did eight nights in Florence the previous year. San Gimignano is small and easy as a day trip from Florence or Siena. Cut down on your one night stays. Uses too much time better spent exploring and relaxing. You could see Orvieto as a day trip from Rome but I haven’t been to Orvieto yet, the one night there may be worth it, I thought others said there isn’t a left luggage there?

At any rate each location change costs you half a day of time you could be visiting something great.

The problems with overnighting instead of daytripping is that you have all your luggage with you. So you have to pack/unpack it and start your transfer at the hotel you're checking out of and end it at the hotel you're checking in to in order to pick up/drop off your luggage.

You aren't giving yourself enough time in Venice. You'll use most of a day going from the Cinque Terre to Venice and another longish journey to Turin. That gives you only one full day to enjoy the city.

Check the travelling times at the Trenitalia website (use the Italian originals for place names, such as Firenze and Venezia), and you'll see that to some of your destinations day-tripping is better than changing accommodation. Rome to Orvieto, for example, can be done in less than 1,5 hrs, even on the extremely cheap Regionale trains (EUR 15,60 for a day return ticket; use the »funicolare« to get you from the railway station to the city above). Florence to Lucca is around the same travelling time and the same price. (Pisa is only 30 mins away from Lucca.) Florence to Siena is around 1 hr on the frequent bus, which delivers you right in the city centre.
Personally I would skip Cinque Terre, which will be extremely crowded, and invest those days in Venice, or in one or two day trips from there to Padova or Vicenza.

As tonfromleiden suggests, the CT will be extremely crowded at peak tourist times. Go to Venice and take day trips to Padova and Vicenza. On the day you go to Torino you could stop for a visit to Verona - dropping your bags at the left luggage spot at the RR station. I also agree with acraven to avoid those one nighters - more trouble than they are worth when you could day trip easily.

Don’t stay overnight in San Gimignano, visit on a day trip. Skip the Cinque Terre and add that time to Venice and /or Lucca which deserve even more time.

On June 19th I would train from Orvieto to Florence and visit Siena and San Gimignano as day trips, that is 7 DAYS is Florence, which is a bit too long and those days can be given to Cinque Terre and Venice. So it would look more like this

June 13th- 18th ROME
June 18th travel to Orvieto - stay the day and overnight
June 19th travel to Florence
June 19th - 23-Florence with day trips to Siena and San Gimignano
June 23 Travel to Lucca and stay the day and overnight
June 24 travel to Cinque Terre and stay two nights (that is all you need)
June 26- 30 travel to Venice and stay there
July 1st travel to Turin

Me and 21 year old daughter just finishing up 16 days. How old are you and what do you like is most important. We had 2 nights in cinque terre and wished it was more absllutely beautiful scenery hiking. Rome hated like NYC crowds w history but if u love history u may love. Venice is magical but again you have to decidd what u like in order to properly answer that question. We had at least 3 nights in each place w exception of C T and again we felt rushed there