We went through Paris to meet our grown daughter in Spain then embarked on a 12 day Mediterranean cruise returning through Paris. For context, it was our first trip to Europe, my wife and I are in our mid-fifties and I did a lot of research including Rick Steves books, forums and audio guides. My thoughts by location:
Paris - stayed both times in the 7th arr and loved this area as it was a mix of locals and tourists, had lots of cafes and was close to major attractions. We used the BigBus hop on/hop off tour which was an easy way to get to all the major attractions in the city. Was somewhat disappointed with Montmartre as it was quite full with tourists.
Seville Spain - stayed here 5 days and our daughter was our guide. Really loved the Triana area which was 15 minute walk to city center, yet full of locals and great cafes. Visits to bullfighting ring/museum and Alcazar castle were worthwhile. Took a day trip by train to Cadiz which was easy (using the train) and fun. Seriously consider staying in Triana if you are willing to walk a bit and looking for a more local feel when visiting this city (used AirBnb for our apartment in Triana).
Barcelona - one day before cruise embarked. Segrada Familia is a must visit. We visited in the late afternoon and the colors from the sun coming through the stained glass windows was amazing. Used public transportation which was pretty easy. Boqueria market was interesting but very crowded. La Ramblas was very crowded and over rated IMHO.
Cinque Terre - had a private guide booked through Viator and BellaItalia which was probably our best day on the trip. Guide was local and very knowledgeable. Handled all tickets and transportation for us. Had fun getting to know him and the area. Got back to the cruise ship on time, which was a slight risk by not using a cruise ship tour.
Rome - the most complex, confusing day of our trip. Attempted to get into town on our own. Ricks notes in the Mediterranean cruise ports about how to get to the train station at Civitavecchia seemed incorrect. Switching between train into Rome and subway was very difficult as station names were different (train/subway) but were actually connected. Audio guide of Colosseum was useful. My advice would be to use cruise or tour to get from the port to the town center and consider a tour guide for a more personal feel for the main sites.
Naples - Amalfi Coast and Pompei - Amalfi coast was interesting and beautiful. Don't attempt to drive this yourself as it was very curvy with buses frequenting the road. Pompei was interesting. Used cruise ship tour here. Decent guide, not as good as personal tour guide though.
Athens - Taxi into town, did Rick Steves Acropolis tour on our own - which was very doable and would do this again. Visited The Plaka for lunch which was good and interesting. The Acropolis museum is worth a visit, and has free wifi in the cafe!
Montenegro - did a 2 to 3 hour taxi tour for 60 euros along the coast to Budva - definitely a good choice. Walked through old town ourselves, which was ok.
Dubrovnik - 1 hour panoramic tour of the town from a taxi, which was a good choice. Beautiful town. Probably the cleanest place on our whole trip.
Venice - Made the mistake of getting a gondola ride from someone who didn't speak English well (don't do this!). Should have confirmed route ahead of time as we went onto the grand canal in our gondola and it was very busy with powered boats. St. Marks square was packed with tourists. Took a few pictures and left.
Next time we'll do it on our own and avoid the big cities (our personal preference), looking for more of a local feel like we got in some of our private tours and our time in Seville.