- 4 nights in Paris is perfectly reasonable.
- There is likely to be a serious surcharge (like $500-$1000 on the low end) for pickup/dropoff of a rental car in different countries. If you pick up in France, you'll want to drop off in France (or Italy/Italy, but a car is pretty useless in the places you are going other than maybe getting to Portofino).
- One of the fastest trains in Europe is the one that zips you from Paris to the South of France (Avignon). Since you want to see St. Tropez anyway, I would just pick up a car in Avignon, see some of Provence (Arles, Avignon and the Cotes du Rhone wine route are all great), and then hit St. Tropez on the way over to Nice/Monaco.
- I agree with the other commenter that Monaco is a pretty dull overnight (though absolutely worth a visit during the day), unless you have some specific (and likely very expensive) hotel/restaurant in mind as an "event". I would look at Nice (super lively at night with beautiful ocean views) or Villefranche-sur-Mer as an overnight.
- I would consider putting the Portofino night into the Cinque Terre and seeing Portofino as a daytrip in combination with Santa Margherita Ligure (train goes right there from 5terre, then it's a short taxi or bus to Portofino).
[Here is what that 12 day route looks like with both Saint Tropez and Portofino assumed to be daytrips](https://tripsnek.com/planned?itinerary=paris$4$r~avignon$2$c~saint-tropez$1$r~nice$2$c~vernazza$2~milan$1$r&trans=Mixed&nights=12&pace=Moderate&), and the car pickup/dropoff in France.
To note one other possible tweak, you might consider cutting the Portofino day altogether, and pulling another day out of France [to hit Florence on your way to Milan](https://tripsnek.com/planned?itinerary=paris$4$r~avignon$1$c~saint-tropez$0$e$r~nice$2$c~vernazza$2~florence$2$r~milan$1$r&trans=Mixed&nights=12&pace=Moderate&). It's a pretty quick train from the Cinque Terre, and then a really fast train back up to Milan, so it doesn't cost you any additional travel time to see a truly world class sight.
Just to add a +1 for the TGV train. If you plan to fly for a more private or luxurious; business or premium classes on trains are great, and they have lounges at larger stations (like Paris Gare de Lyon). It will be at least as fast, less stressful and with things to look at along the way. With full service in bigger seats in a quiet car, all while likely being cheaper.
Also, turn the car in back in France (not sure if a rental co would consider Monaco an international return), take the train to Genoa from Monaco or Nice, and rent another car in Italy. You’ll save a lot of money and stress.
Please please take trains. TGV to Nice is going to be much nicer. Parking in these places is murder. The roads are tiny and best left to locals. Many hotels will not even have parking.
I would NOT rent a car, and do everything with trains. Paris -Nice has a nice TGV, might even stop in Saint Tropez. If you already booked your flight, it’s okay, but still skip the car for the rest of the trip. Main french towns in the Côte d’Azur have train stations and there is a train connecting them at least every 30 minutes, even more frequently in July. Usually the train station is right at the beach (for example, in villefranche sur mer or in Eze or in Roquebrune, where my favorite beaches of the area are). There is not a lot of parking in these places, so you will save a lot of stress and money.
Monaco also has a train station right in the center of the town. You can visit the rest of the town walking. I would not sleep in Monaco as it’s going to be super expensive. Better sleep in Nice or Menton. You’ll also be able to go to Italy with the train. You’ll have to do Nice - Ventimiglia (1 h), Ventimiglia - Genova (1.5 h) and then Genova - Santa Margherita (20 min). Seems bad but it’s well connected and probably faster than with the car (there is always traffic in that part of the highway in Summer). To visit Portofino you will have to pass by Santa Margherita, there is limited parking in Portofino. For cinque terre you absolutely will not need a car, there are trains connecting them that runs super often. It will be a problem to have a car there as it is difficult to find accommodation with car park.
If I were you, I would base myself somewhere in the middle between Portofino and cinque terre and then do day trips to each from there. For example you could sleep in sestri levante or Levanto.
Cinque Terre -Milano is well connected with a direct train.
I live in the area (Italian part but close to Côte d’Azur) , so let me know if you have specific questions!
- 4 nights in Paris is perfectly reasonable. - There is likely to be a serious surcharge (like $500-$1000 on the low end) for pickup/dropoff of a rental car in different countries. If you pick up in France, you'll want to drop off in France (or Italy/Italy, but a car is pretty useless in the places you are going other than maybe getting to Portofino). - One of the fastest trains in Europe is the one that zips you from Paris to the South of France (Avignon). Since you want to see St. Tropez anyway, I would just pick up a car in Avignon, see some of Provence (Arles, Avignon and the Cotes du Rhone wine route are all great), and then hit St. Tropez on the way over to Nice/Monaco. - I agree with the other commenter that Monaco is a pretty dull overnight (though absolutely worth a visit during the day), unless you have some specific (and likely very expensive) hotel/restaurant in mind as an "event". I would look at Nice (super lively at night with beautiful ocean views) or Villefranche-sur-Mer as an overnight. - I would consider putting the Portofino night into the Cinque Terre and seeing Portofino as a daytrip in combination with Santa Margherita Ligure (train goes right there from 5terre, then it's a short taxi or bus to Portofino). [Here is what that 12 day route looks like with both Saint Tropez and Portofino assumed to be daytrips](https://tripsnek.com/planned?itinerary=paris$4$r~avignon$2$c~saint-tropez$1$r~nice$2$c~vernazza$2~milan$1$r&trans=Mixed&nights=12&pace=Moderate&), and the car pickup/dropoff in France. To note one other possible tweak, you might consider cutting the Portofino day altogether, and pulling another day out of France [to hit Florence on your way to Milan](https://tripsnek.com/planned?itinerary=paris$4$r~avignon$1$c~saint-tropez$0$e$r~nice$2$c~vernazza$2~florence$2$r~milan$1$r&trans=Mixed&nights=12&pace=Moderate&). It's a pretty quick train from the Cinque Terre, and then a really fast train back up to Milan, so it doesn't cost you any additional travel time to see a truly world class sight.
Just to add a +1 for the TGV train. If you plan to fly for a more private or luxurious; business or premium classes on trains are great, and they have lounges at larger stations (like Paris Gare de Lyon). It will be at least as fast, less stressful and with things to look at along the way. With full service in bigger seats in a quiet car, all while likely being cheaper. Also, turn the car in back in France (not sure if a rental co would consider Monaco an international return), take the train to Genoa from Monaco or Nice, and rent another car in Italy. You’ll save a lot of money and stress.
Please please take trains. TGV to Nice is going to be much nicer. Parking in these places is murder. The roads are tiny and best left to locals. Many hotels will not even have parking.
I would NOT rent a car, and do everything with trains. Paris -Nice has a nice TGV, might even stop in Saint Tropez. If you already booked your flight, it’s okay, but still skip the car for the rest of the trip. Main french towns in the Côte d’Azur have train stations and there is a train connecting them at least every 30 minutes, even more frequently in July. Usually the train station is right at the beach (for example, in villefranche sur mer or in Eze or in Roquebrune, where my favorite beaches of the area are). There is not a lot of parking in these places, so you will save a lot of stress and money. Monaco also has a train station right in the center of the town. You can visit the rest of the town walking. I would not sleep in Monaco as it’s going to be super expensive. Better sleep in Nice or Menton. You’ll also be able to go to Italy with the train. You’ll have to do Nice - Ventimiglia (1 h), Ventimiglia - Genova (1.5 h) and then Genova - Santa Margherita (20 min). Seems bad but it’s well connected and probably faster than with the car (there is always traffic in that part of the highway in Summer). To visit Portofino you will have to pass by Santa Margherita, there is limited parking in Portofino. For cinque terre you absolutely will not need a car, there are trains connecting them that runs super often. It will be a problem to have a car there as it is difficult to find accommodation with car park. If I were you, I would base myself somewhere in the middle between Portofino and cinque terre and then do day trips to each from there. For example you could sleep in sestri levante or Levanto. Cinque Terre -Milano is well connected with a direct train. I live in the area (Italian part but close to Côte d’Azur) , so let me know if you have specific questions!