Big three for first timers...One thing to do !

If you are looking for a fun foodie thing to do in Rome, take the Testaccio food tour. Testaccio has a long food history in Rome, being the area where foodstuffs were unloaded for use in Ancient Rome, and there’s a great outdoor market there, which you will tour. I took several food tours in Rome and enjoyed this one the most.

Well, it's probably already on your list, but just in case you are hesitating because (many reasons)...splurge and do the gondola experience in Venice. Sure, it's a tourist trap. Yeah, it's expensive. And yes, it'll feel cheesy, at least initially. Even though it's all that (and it is), it's still worth doing.

Our gondolier saw me and my wife happily gazing into each others eyes, and sang out: "everybody is happy in the goldola!" He was right: I looked around at everybody else in all the other gondolas, and every single person of every age and nationality were all grinning from ear to ear.

Do it, you won't regret it.

In Venice, take Alessandro’s Ciccheti Pub Crawl. Food, vino, great host. Highly recommended by Ricksters who’ve visited La Serenissima.

Take the vaporetto to San Giorgio just across the basin from San Marco and explore the grounds of the church and monastery (the boat lets you off right at the foot of the steps of the church), and enjoy the coolness of the church, and make your way back to rear of the church on the left side and pay the very small contribution in the bookstore and go up the lift/elevator to the top of the bell tower.

Circle round the top until you are lined up with San Marco and just drink in the view of the whole of that part of Venice. Admire the ballet of the boats and the rooftops. See how many leaning bell towers you can spot.

Rome - walk around the Foro area, St. Peter's after dark. Have a drink in front of the Pantheon. Have a late night gelato and sit by the Bernini fountain in Piazza Navona.

Florence - definitely go to Siena (the car is spelled with 2 n's). (I like 3 nights in Florence and add the extra night to Venice)

Venice - go dancing in Piazza San Marco at night then take a vaporetto around the Grand Canal. Now that's romantic!

Saving for future ideas. Is there a way to bookmark posts? Thanks.

Florence - walk up the hill to St Miniato late afternoon and enjoy the view. Go into the church for the chanting of vespers.

Venice - yes, a gondola ride, in the evening. So what if people think its touristy. Its a centuries old tradition.

Rome - a gelato and an evening walk.

Lynn you can bookmark a post like any webpage. Maybe make a bookmark folder entitles Italy and save it there in your browser.

In Venice, visit the Querini Stampalia Foundation, at Campo Santa Maria Formosa. A good place for a bite of lunch (though not a foodie paradise), and a lovely garden.

My favorite thing in Rome (among many favorite things) was a private, after hours tour of the Vatican Museum & Sistine Chapel. Just being there without a crowd was fabulous, but seeing this with a guide made the experience more special for me. (Not Catholic. Just love art & history.) The guide was able to put everything in context which made these works come to life for me.

One of my favorite things in Florence was getting lost. (This happens every time we travel, but this time it really worked out well.) We wound up having a late lunch at a place we never would have discovered otherwise, and it was just a very serene and peaceful experience after pushing through tourists earlier that day.

In Venice, I agree with the gondola ride suggestions. And if you're feeling like a show, watching the people in St. Mark's square in the evening is also quite enjoyable. (It can also be quite expensive, but that's another story entirely.)

In Venice you could also consider rowing lessons with Row Venice. You’ll learn to row a gondola and will, after some practice, actually row on the Grand Canal. If you take the late afternoon lesson you’ll also row to some canalside bars and enjoy wine and snacks. It was a favorite memory in Venice.

This is, I know, a very boring answer but "if you have to chose ONE thing to do in each of the cities above what would it be?" then I would do what most of us do for our first (and subsequent) trips. In Rome go to the Colosseum & Forum, in Venice go to St Marks square and visit the buildings around, plus take a water bus along the Grand Canal. I didn't think Florence was one of the "big three" (surely it is Naples), but after seeing the David statue, my favourite thing was walking around the pretty centre, especially the other side of the river.

I realise I have a fairly flexible interpretation of the word "one"!