Skipping a leg of our flight

You can skip the last leg, assuming of course you haven't checked luggage. But you can't skip the first leg, as the rest of your reservation is automatically cancelled and you will NOT be allowed on the second flight. All airlines do the same for first or intermediate legs. You could check rebooking fees to fly into Florence but out of Zurich, and take the train from Florence to Freiburg. You should be able to see this in the terms and conditions in your ticket confirmation email.

It's unclear exactly what your whole itinerary is. You're about to be told that skipping a segment will cancel the remaining ones, but it ain't necessarily so. Call the airline. If you don't like the answer, ask when initially checking in. If you don't like the answer check at the end of each segment up to the one you want to skip. If you get a good answer, make sure that it's booked that way and you're not left with just a verbal okay. It can sometimes be done.

Here is the standard answer - Skip a leg and there is a high probability that the balance of the ticket will be cancelled. You can get away with skipping the last leg because there is nothing left to canceled. Son and girlfriend got away will skipping the second leg, as you are suggesting, by faking an illness. Too ill to board the second leg but did so in front of the gate agent who give them a mild warning about not being able to us that portion of the ticket later, would have pay a change fee IF they do so, etc. Two days later checked and their return flights were still intact. So, as Ed suggested, it can be done but you cannot just walk away and hope for good things to happen. You absolutely cannot skip the first leg of your return flight because they could give it to a standby passenger who would have your seats on the next flight.

Do as Ed suggested and talk to the airlines, don't risk losing any other legs of your flight by doing it on the sly.

I've done it before. I booked a ticket to Cologne - non-stop to Schipol, Amsterdam on KLM and a connection on Lufthansa for the hop to Cologne. After booking the ticket, I started pricing rental cars/leases. It turned out leasing out of Amsterdam was the best deal. We flew to Amsterdam and stayed two nights, rather than catch the connector, before driving on to Cologne. The flight home, out of Rome, was with KLM. Missing the connection didn't seem to make any difference to them. Maybe because it was the last leg, or maybe because it was a separate ticket on a different airline. This was in 2005, airlines seem to police this much more than they did just a few years ago because some people gamed the system to get better prices by booking legs they had no intention of flying. The worry is that canceling the remainder of your ticket, may include your flight home. I still think you can call and confirm your flight home without too much trouble, after you have skipped your flight, but I think it's important to call them before you show up at the airport expecting to board your flight home (probably the sooner the better).

Basically you would be violating the terms of your Carraige Agreement (all the fine print on the ticket). You could risk it, but the risk is that you may wind up buying a new ticket back to the US. Technically, even skipping the last leg is a violation, giving them the right to come back and charge you the difference in ticket costs, or at least a change fee, but there is a lot of anecdotal evidence out there that they do not bother. You could see what the change fee would be to do what you want, it may not be that bad.