I was thinking of starting a similar topic, as we will be bringing our 9 and 11 year-olds to Italy for the first time this summer. We took their older brother on a similar trip 12 years ago, and we are hoping to recreate some of the magic from that first trip. I too would love to hear anyone's can't miss activities with kids, but we already have a pretty packed itinerary. Here are some of the things we're planning on doing:
Collodi's Pinocchio Park. It's in a little town just North of Florence that has statues and exhibits that walk through the Un-Disneyfied version of the story of Pinocchio. A lot of climbing and running around for the kids and a must do for us.
In Rome - Gladiator School
In Florence - Pizza and Gelato cooking class at Food & Wine Academy
In Venice - Mask making class
I think a late afternoon/early evening stop in Siena's Il Campo is a must. On our first visit, about a half dozen boys, including our son, played a make-shift soccer game with a beach ball. I think it literally hit everyone in the square at least once. No one cared.
In Venice, there is a tiny park in the Southeastern part of Castello overlooking the lagoon. Now this was 12 years ago, but there was some old and rusty play equipment, which probably would have been off limits in the States. But every day neighborhood kids would play there in the late afternoon. Our son had a great time interacting with local kids there.
I'd love to hear any other suggestions.