Welcome to Fodors Abshim.

My initial thoughts are that you are doing a lot of rushing about on trains with the odd city thrown in, and I'm not too sure what you want to see, history buffs no, but do you like art? Milan is interesting for a big church, some shops and a small museum but it does have a big football team, what is it that attracts you here?

Without understanding what you like to do I suggest the following itinerary;

I think if you are flying into Venice and out of Milan then look at
Venice 2 days
Florence 4 days (side trip to Sienna and Pisa)
Milan 1 day (spend most of it in the main square)

Looks like you are trying to do Venice, Rome (or Florence), and the lakes all in one week. They are all fabulous in their own way. Remember, you'll lose about half a day each time you switch locations, so this really gives you very little time to enjoy each. The plan above is a good one, or you may want to pick two of the three places on your list so you will have enough time to really enjoy them. Also, Florence is smaller than Rome, and more about art than history, so it might be a better choice for you with limited time.

Do you have your plane tickets? If so, where do you fly into and where do you fly out of? If you haven't bought them yet, do look at open jaws flights, into one city out of another. This is called multi-city on the airline web sites and will save you the time and money spent backtracking to your arrival city.

It looks like you fly into Venice. You could fly out of Athens.

But how many days -- or better nights -- do you have?

And does your family include children? If so, of what ages?

If you're not interested in history, what are you interested in? Art? Scenery? Food?

Florence is best for people interested in Renaissance art, architecture, history. Venice can be enjoyed by all ages just for its unique setting on canals: no cars, no trucks. You walk or take a boat, a vaporetto/bus or a taxi boat. The Italian lakes are lovely but you don't need to go all the way to Milan and Lake Como. Lake Garda is nearer to your route, won't take you so far out of the way so will use less time.

Thanks for the replies guys!
In answer to your questions: we're 5 of us including an adult sister, my parents and my husband. We haven't booked our tickets but will probably fly into Venice and out of Athens. We enjoy food, (and we're vegetarian), would love to see some of the art and ancient sites but probably just a taste of it would be good too.
So here's what I think:
How about I do 2 nights in Venice, 3 in Florence (check out Siena), 2 in the cinque terre (just looked at pictures and it's utterly beautiful), and 2 in lake Garda maybe if it's close by. I could do athens for 2 nights and satorini for 3.

I'd have a look at a map to understand where each of these places is so you get the sequence right. Google maps will do.

Cinque Terre requires a fair bit of hiking so I hope the family are up for it.

So where are you flying to Athens from? Are you still going going to Rome too?

Generally we suggest flying into Athens and then directly on to Santorini. After Santorini fly back to Athens and spend your two days then.

Yeah that's the plan for Greece. Satorini and then 2 nights in Athens.
Italy probably requires a bit more research what with travel times, distances and locations.
From what I understand lake Garda can be done from Venice, and the cinque terse after that? O
And then onward to Florence?
We're flying in from India while my husband will join us from London.