I always said I would never visit Italy in the heart(heat) of summer, but.....

We’ve done a more extensive trip only across northern Italy. With 10 days pre Siena and your desire to be in the Dolomites maybe something like this would be good with a day trip thrown in based on your family’s interests.

Fly into Venice (2 nights)
Train/bus to somewhere in the Dolomites (3 nights)
Train to a lake (3 nights)
Train to Siena (2 nights)
Fly out of Pisa, Florence, wherever…

Keeping it relatively cool in July!

I must warn you that I had a great deal of trouble finding affordable lodging on Lake Como (beginning August 30) and in Bolzano (which is down in a hot, low-lying valley, not up in the mountains) for early September.

Verona is a possibility, but the opera festival drives up hotel rates there. Padua has some interesting sights and usually has much less expensive lodging options. A couple of places between Verona and Venice recommended by others are Treviso and Bassano del Grappa; I haven't been to either yet. Edited to add: I should have mentioned that those two towns are north of the main Milan-Venice rail line.

If you flew into Milan, you could consider time in Bergamo and/or Brescia before going to the Dolomites.

Alternatively, you could hit the Dolomites and then go to Bologna, which offers some very attractive side-trip options.
 
I adore Ravenna, but I'm not sure it makes much logistical sense in combination with the Dolomites and Siena on a short trip.

Personally, I wouldn't recommend flying to Venice and spending just two nights there. You'd only have one full day in the city, and some members of your family might still be off-kilter on that one day.

Ortisei in the Dolomites is a great location and the lift served hiking would appeal to those less physically inclined as well. Three or four days there would be perfect followed by a couple days around Lake Como before heading to Siena.fly in and out of Milan for some additional big city time if you need it.

How about Lake Garda in addition to Dolomites? I was going to have 5 nights in Dolomites and 3 in Lake Garda this year but will have to wait. But very doable.

I would also suggest looking into Lake Garda - particularly Malcesine. It is touristy but that's because it's charming and in particular it has a cable car up to Mount Baldo for hiking around or back down to Malcensine or up if you want a real challenge. The Malcensine to Mount Baldo hike is 6.6 miles and gains about 5,500 feet. You can also use the upper lake ferry to visit the other towns around the lake.

Have a great trip,
=Tod

Thank you all so very much! Your suggestions have saved me hours of my initial research. I'm thinking of adding 1-2 days onto the itinerary.

How is the drive to Ortisei - very scary? I see that there is a train from Venice but I've read in so many places to rent a car for the Dolomites.

It seems like we would either fly into Venice or Milan. I think Venice fits us better than Milan if I had to choose right now....although the report of heat and crowds does give me pause for Venice. (I do understand we will have crowds probably everywhere).

There's good bus service from Bolzano up to Ortisei; that's the way I traveled, and I don't remember a particularly challenging road. Before you get to Bolzano you are not in the mountains, so I think it would only be traffic you'd need to think about.

I believe you'll find a lot of people have been to the Dolomites without a rental car, though if you have very specific areas where you want to hike and there are few or no bus connections, that would be a good reason to have a car. But there are lifts as well as buses to get you to a lot of good hiking areas.