Been to Rome with teens. We were there for 4 ½ days, and thought we'd take a day trip to Ostia Antica but ended up not (too much to do in Rome). We did spend an afternoon in the Borghese gardens/park after our morning tour of the museum. We ate a picnic lunch that we bought at a restaurant there, and then rented segways for the first time (purely with the kids in mind), and also bikes. This was our sort-of non-touristy day where we just did things we thought the kids would like. Other hits were the underground and third tier tour of the Colosseum (we did the one through coop culture), and an abbreviated (2 hours instead of 3 maybe, but our youngest kid was 9 then) tour of the Vatican museum, St Peters and dome climb. We visited the Capitoline museum on a whim and all liked that. We stayed in the Trastevere neighborhood and really liked that. We did spend a separate/additional 3 nights/2.5 days in Florence, which we also loved (so 8 nights total between the two), and there we visited the Uffizi, and Academia (to see David), among others and walked extensively.