Thank you everyone for the very insightful and honest advice! We really appreciate it!

We took to heart what everyone said and here's what we are planning to do:
- Unfortunately, it's not possible to change our flight now without paying extra. The airfare was over 5K already, so we are just keeping it as is.
- We ditched the car and we'll travel by train from Geneva to Italy, visiting Lake Como (3 days, July 19-22), and Venice (3 days, July 22-25). Each of us will have a medium sized suitcase and a carry-on.
- We'll take a ferry from Venice to Croatia and we'll rent a car there, as we already booked a holiday home that has parking (3 days, July 25-28). We'll take day trips in Croatia (just the Istrian peninsula).
- We'll take the ferry back to Venice and we'll check out the Lake Garda area and stay there for the next days (July 29-July 31). This lag of the trip is not crystal clear yet, but we love driving, so maybe we'll rent a car in Venice and just drive there...not sure...or, we can take the train from Venice and rent a car at the lake just for the 3 days, so we are more mobile around the Lake Garda area...
Take the train back to Geneva (Aug 1), stay there Aug 1-Aug 2, and take time to check out the Large Hadron Collider CERN (France).
Travel back to the US on Aug 3.

So, bottom line we reduced the number of places we'll stay at so we can enjoy the sights (already booked a few) and we'll drive in Croatia and maybe from Venice to the Lake Garda areas and then from there to Milan to catch the train back to Geneva. It will be a very new experience for us, as we always drive around everywhere we go. Last year we drove in Turkey for 2 weeks, which was amazing...yes, we stretched ourselves, but we toured many amazing historical sites, drove from Istanbul down to Marmaris and took the ferry to Rhodes and stayed there for a couple of days, too. The difference is that it was in one country and we flew from Antalya back to Istanbul. A year before we went on a Germany-Austria-Hungary road trip fro 2 weeks, which was also amazing. BUT: we never drove in Italy/Switzerland/Croatia before. AND we never did a "train vacation".

This is where we are at the moment...we are in the process of finalizing accommodations and we'll see if we can get a private tour guide to do a day tour in Venice for us.

This sounds much more manageable and enjoyable! And while it’s a bit more rushed than my own preferred travel style, it does leave ample time for both sightseeing and relaxing…
I think you all will enjoy riding the Italian trains. Very comfortable and efficient way of getting around, and with views…
I hope your trip goes smoothly and you all return with happy memories of Italy and Croatia.

I'd drop Croatia. Too much time/trouble to get there for just 3 days. You could definitely use more time in Venice.

The town of Como is convenient if traveling by train, but it's the least interesting, least scenic town on the lake IMO. Staying in Como means spending more time riding ferries and buses. You should study the ferry and bus timetables and organize your sightseeing to maximum your brief time there.

Rather than time on Lake Garda (you will have already experienced Lake Como), I would take the train from Venice to Bolzano (3 nights). Visit the Ice Man exhibits at the archeology museum in Bolzano, take the cable car from Bolzano to the high plateau above the town, maybe make a day trip by bus to Ortisei. From Ortisei, you can take another cable car ride up into the mountains and see a (tiny) bit of the Dolomites.

I'd agree with you but it seems they have already booked a vacation rental there and its probably too late to cancel . . .

+1 to what many people said. You don't need a car in Italy unless you are looking to spend a bunch of time in Tuscany driving around. Trains are easily accessible, quick and pleasant.

Have you considered going to Florence from Geneva, if you have not been there? Cinque Terre is a nice side trip from there too. You may be able do that on the way to Croatia and visit a lake on the way back.

I assume that you have a good reason to go to Croatia and that you are good sailors since you have decided to take the ferry. I know people who have done that crossing and without exception they all had a rough, somewhat unpleasant crossing, so remember to take some sea sickness pills. I did the longer crossing between Ancona and Split overnight in July many years ago and nearly everyone was sick. I think I was one of 4 people eating in the restaurant.

If you have not already booked catamaran tickets, consider taking goopti instead. It is a shuttle, shared or private, that can take you from Piazzale Roma in Venice to close to wherever you are picking your car up. I’ve never heard about too many people getting seasick on the catamaran, but if the weather is bad they will put you on a bus. Taking a shuttle will allow you to go on your schedule, and not arrive in Istria at night or have to leave Istria super early. Going back, you can book goopti to an Italian airport or to Trieste train station to pick up the car.

https://www.goopti.com/en/

Yep. Fairly typical "is this crazy" inquiry posted less than 2 weeks before departing... I'm not sure our comments are worth much. Flights can't (or won't) be changed, lodging is likely booked, can't be changed or alternatives are no longer available. Ergo, the plan is the plan. At least the idea of driving after landing was abandoned.

Is this a joke?

I gu

I guess you didn't see the post where they had pared down the itinerary.

Hi, that sounds like a much better plan! I would add more days to Croatia instead of the Lake Garda area as there is so much to see in Croatia, but the itinerary works as is too. Just don't know anything how the ferry crossing would be!

Wow, sage advice from well know travelers on Fodors. My two cents is learn to live with a carryon and backpack for 2 weeks. Watch packing videos on managing folding or rolling. You ae going in and out many trains, rental car...Try it for one trip and you will be convinced. Have a great trip.

Similar route to the one we did on our honeymoon, except start and end in Frankfurt, and Croatia was Yugoslavia.
Did it in five weeks.

Your latest itinerary looks fine. I would just add that before you decide to drive in Italy you may want to check out the Zona Traffico Limitato for where you want to visit. There are ZTLs around Lake Garda, Verona and most historic town/city centre that restrict driving through "pedestrian" zones of many of the lakeside villages. You will see vehicles belonging to residents driving through as they have permits, other vehicles have to park or take an alternative route. If you enter a ZTL restricted area, you will be fined. Each ZTL has its own time restrictions, usually lifted overnight.

You can explore Lake Garda by ferry and bus. Road traffic can be heavy around the lake.

The OP is more than halfway through their trip, so I doubt they're looking here for more suggestions/ideas. But reminders about ZTLs in Italy (they're everywhere!) are always worth repeating.

Yep - the travelnuts will be flying home in four days. Sometimes on long-ish threads I think people just don't page back up to re-read the OP therefore missing the dates, and just carry on posting their sage advice no matter if its useful or not ;)

The only remaining question: Will there be a trip report?

If they stuck to the last mentioned plan, they're at Lake Garda today and tomorrow... where the feels like temp is 93F. Ugh.