port to vatican, from vatican to coliseum back to port

There are about 2 Regionale trains every hour to Rome and it takes between 45 and 55 minutes on these and costs less than 5 euro pp. Another advantage is it stops at Roma San Pietro which is about a 10 minute walk from the Vatican. Vatican to Colosseum is by subway, changing at Termini station. When returning, go back to Termini and take the train to Civitavecchia from there. There are a few fast trains that do the route for more money.

Four people can also share a taxi from St. Peters to the Colosseum for under €15 (approximate metered rate).

To make this work, you are going to have to hustle. Even with docking at 0800, I doubt if you can get to the train station before0900. It is an hour to San Pietro on a Regionale train. To save a lot time, buy your Regionale train ticket at any train station prior to arrival in Citiv... The line to buy tickets at Civit... can be very long especially if more than one cruise ship is in port. Buy eight tickets. There will be shuttle from your ship to the port entrance. From there it is a 15 min walk to the train station along the beach. You can see the train station from the port entrance. At the station check the departure board for the next R train to Rome and platform number. On the back the station is a yellow/blue/green validation box. Validate your tickets - time stamp. Go to the platform and get on the next R train. No reserved seats so you may stand till something becomes available. The cruise passengers overwhelm these trains. Get off at Saint Pietro - short walk to Vatican. Should easily make the 1130.

With four it will be cheaper to take a taxi to the Colisseum. And the same for Termini. I would try to be in Termini between 1600 and 1630. Same procedure in reverse. Validate ticket and get on the next R train. In the past the Civit.... trains have use the far, very far, right hand tracks. It is good hike inside Termini to tracks 25,26 (I think that is the number but double check with the departure board) . Another hour to an hour 15 back to Civit.... If departing at 1900 you will be required to be on board by 1830.

RoninRome.com has a description of getting to Civit... Might want to review that.

I think you will need to buy four one-way tickets to Roma San Pietro and four one-way tickets to Civitavecchia - direction matters when you validate. Don't validate all eight tickets at once.

Count on at least half an hour between Saint Peter's Basilica and the Colosseum. Taxi stand is just outside the colonnade at Saint Peter's.

Since the fare difference between San Pietro and Termini is like 0.40 euro, It would be easier to buy 8 tickets for the route Civita-Termini, you can just get off the train at San Pietro and Trenitalia will thank you for the extra 1.60 euro.

I really appreciate all your tips and comments. I think is going to be really busy so all those tips are truly appreciate. Can I pre buy that regionale train ticket in Venice? Can I pre buy the 4 tickets from Citiva.. to San Pietro and the 4 tickets from Termini to Citiva.. and pre pay with a credit card?
What are your thoughts?

Sure, and that is smart. Use the train station in Venice and your card credit will be accepted by a live person. A Regionale ticket is valid for 60 days from date of purchase. BUT, it must be validated (time stamped) and then it is only good for five hours in the direction of travel stated on the ticket. Find the validation box in Venice so you know what they look like and watch someone validate their ticket. Last year there was a validate box on the back side of the station in Civit.... before the first set of tracks. The boxes can be in the underground passageway between tracks. And don't forget to validate and keep the ticket handy. The fine for not having a validated ticket is 50 or 60E on the spot. Pay now or go to jail. And make sure you are on an R train. There are IC (Intercity) trains on that route but they all require a seat reservation.