My wife and I will be visiting family in Padova and Sicily for two weeks this fall. We would like to decide later when to fly between regions. Is it realistic to plan as we go and walk on to a flight? What should we expect to pay with no reservation lead time? The airports we'd be using are vce & cta. We are one carry-on bag travelers and are pretty flexible. Thanks for any help.

Not a good idea in my opinion. It takes time and effort to get to the airport and just showing up is a risky hassle and, yes, you will most likely pay much more. As you probably know, airfares swing wildly so there is no one answer to your question.

It is no easier or cheaper to "walk on" to a flight in europe as compared to North America.

I may be in the minority here, but when people say they want to decide as they go how long to stay in an area or where they want to go next, I sort of feel sorry for them. It seems to me that such thoughts show a total lack of planning. If you've studied and read about the areas you plan to visit, it shouldn't be hard to figure out how much time it takes to do that. Arriving at a place without knowing what you want to do, just means you'll probably miss most of what there is.
Now of course, it is always possible that when you get to a certain place, despite a lot of planning, you really hate it and want to leave earlier, but that should be the rarity.
I just can't imagine going somewhere and at the last minute deciding where and when to fly next. My travel time is too valuable to spend a day or more trying to book last minute travel reservations, searching for a hotel in a city where you have no reservations, and spending half a day at an airport waiting for the next available flight. But hey, if that's how you enjoy yourself -- go at it!

When you are visiting relatives, sometimes you have just got to make a hasty departure!

A hasty departure would be even easier if you just told them, "sorry folks, had a great time, but we're out of here - have a plane to catch."