Finalized plan help

Don't book the train from Rome to Florence in advanced not until you are sure that your flight is arriving on time to catch it.
If you do book in advance give yourself 4 hours between the arrival of the plane and the departure of the train.

You can book the trains from Vernazza to Rome in advance.

I would use Trenitalia.com or thetrainline.com. The latter lists and sales train tickets on both Italotren and Trenitalia.

If you are not averse to hiking and seeing great views from high up there is a nice hike from Monterosso to Vernazza.
I have gone to Italy the last two years in May it is warm in the day in the 80's or high 70's that time of year.
It can get chilly at night. Not really sundress weather until after June through August

Sounds like a lovely trip. My husband is from Lucca and I've been 4x (and will be back this May 24-27). Having bikes is wonderful -- plan on spending mornings or afternoons cycling around the wall. Pack a picnic and stop under a shady tree, read a book, wiggle your toes in the grass. You can drop down into the town for gelato or pizza. We always go to Pizzeria Felice at Via Buia 12 (when my husband says it I see it written as Bool-ya). His parents went there on dates in the 1950s. It's small, no table service, pizza by the kilogram. But soooooo tasty. An authentic Tuscan experience. To me, Lucca is a lot of hanging out (well, a lot of that is my husband visiting friends and family and eating 5 hours a day). If the weather is really hot, jump on the train and hit the beach at Viareggio.

Modest wear for churches. Some more strict than others, but typically, both genders covered knees and shoulders. A big shawl/scarf will work for shoulders. Some like zip off pants and carry the lower parts until required in church. Crop pants or similar. Some European men wear the shorter hemmed pants.
Many towns still have free water fountains to refill reusable water bottles vs paying for water at meals.
My tip website is Wunderground. It provides historical weather temps and precipitation as well as current weather. Find your town, look at 'calendar' and the choose month and year. I look back past 4 years.

Thanks so much everyone!

Does anyone have experience or knowledge on what sights we need to prebook and how to do so?

We would like to go to the Vatican and colosseum when we are in Rome and I’ve heard they need to be prebooked but don’t know where to start

You need a guidebook. Rick's books don't cover all the little towns in Italy, but the places he does cover, he covers quite well, including how to pre-purchase tickets.

Check your local library for any current guide book to direct you to official booking sites. Uffizi Gallery, galleria d'academia for David in Florence. Vatican,colesseum should be booked ahead. From their official website , it should lead to official bookings. Don't use third party sites.

Hi thanks

I’ve looked at guidebooks as stated in my original post. :)
Still have the same questions.

Also no local library near us unfortunately.

Was just hoping someone could point us in the direction of the correct sites.

For the Colosseum use this.

https://www.coopculture.it/en/colosseo-e-shop.cfm

The site can be confusing, so if you have questions come back here.

For the Vatican, thus is the official site.

https://biglietteriamusei.vatican.va/musei/tickets/do?weblang=en&do

For tours try The Roman Guy.

Hi

Attempting to buy tickets from the suggested coop culture site but am incredibly confused as to what we can and cannot buy

We will not be getting our own private tour guide

There are 2 of us. We would like to try to go Saturday Sunday or Monday.

What tours were you suggesting with the Roman guy??

Thanks in advance

Definitely do the duomo climb. Also the climb at the St. Peter's in Rome. Have gelato every chance you get, not kidding. Pantheon and Trevi Fountain in Rome are not to be missed.

For the Colosseum, it depends on what YOU want to do.

http://ecm.coopculture.it/index.php?option=com_snapp&view=products&catalogid=A4CC149C-BEE1-5773-5E59-01675F3EA81C&snappTemplate=template3&lang=en

It's a combo ticket, which includes the Colosseum, Roman Forum and Palatine Hill.

Just visit the Colosseum...no guide € 12+ 2 reservation fee. €14

With a guide? Additional charge. € 14+ 5 =€19

Colosseum with Underground guided €23

Make sure you book any guided tour in English.

https://theromanguy.com