I'm going to repost some of my past suggestions for Paris, Rome, Venice and Florence. I have tried to go beyond the most typical tourist attractions, which will be adequately covered in any guidebook. I hope you find this useful. If you would like specific hotel and restaurant recommendations, please feel free to ask.
Paris:
1) Have lunch or dinner on the Île Saint Louis, which is right next to the Île de la Cité in the middle of the Seine. After dinner, walk the circumference of the island. Sit on the quai, eat ice cream and watch the world go by.
Île Saint-Louis
2) Go to the Marché aux Puces de St. Ouen, which is the original and largest flea market in the world. All sorts of interesting things and people to be seen. You can get there easily on the Metro.
Marché aux Puces de St-Ouen
3) Visit the Paris Catacombs. It is both humbling and fascinating. Six million souls, all alike in death. The Resistance hid out there during WWII. The entrance is at Place d'Enfert-Rocherau.
Catacombs of Paris
4) Visit the Musée de Cluny, which was a Roman bath, then medieval abbey and now a museum. They have interesting exhibits, including 600 year old tapestries.
Musée national du Moyen Âge
Rome:
1) Tour the catacombs, and then San Clemente, which is three churches built one atop the other, and all atop a Mithraic temple. This is the tour we took.
Rome Catacombs Tour & Basilica di San Clemente | Dark Rome Tours
2) Visit the Largo di Torre Argentina, which has the remains of four Republican era temples and is where Julius Caesar was assassinated. It also has hundreds of stray cats, who sometimes will let you pet them if you need a feline fix.
Largo di Torre Argentina.
3) Visit the Forum Boarium, the former cattle market of Rome, where there are two very well preserved Republican era temples and the church of Santa Maria in Cosmodin, on whose front porch sits the Bocca della Verità made famous in the Audrey Hepburn film "Roman Holiday".
Forum Boarium
Bocca della Verità
4) Walk to the top of the Janiculum hill. The panoramic view of Rome from the Finnish embassy is fantastic. On the way, stop at Bramante's Tempietto located within the courtyard of the church of San Pietro in Montorio. Also stop at the strikingly beautiful, white Giancolense Ossario Mausoleo, which is a monument to the Garibaldini who defended Rome against the French in the siege of 1849.
Janiculum
San Pietro in Montorio
Gianicolense Mausoleum Monument (Mausoleo Ossario Gianicolense)
Florence:
1. Go to the opera at St. Mark's English Church. It is a very intimate venue and we enjoyed a lovely performance of
Carmen there. The proceeds go to help orphaned children in Africa.
Opera at St Mark's Church Florence, a unique intimate venue
2. Stop in the small, simple Church of Santa Margherita dei Cerchi, in which you will find the crypt of Dante's great love, Beatrice Portinari. The lovelorn leave notes and letters for Beatrice in the two wicker baskets that sit next to her burial vault. If you are bold enough, you can pull some out to read. Written in all all hands and every language, they pour out their hopes and dreams to a young woman who died over 700 years ago. The most poignant one I read said simply (in English), "I just want to find someone to love me."
3. If you are in shape to climb 463 steps, you can go up to the the top of the Duomo. Climbing through the rabbit warren of tunnels between the inner and outer domes, you can appreciate the genius of Brunelleschi the architect. Twice on the way up, the stairs stop and you are on one of the two inner balconies that circle the great dome at its base. The congregants below look like ants and you have a spectacular view of Vasari's painting that covers the inner surface of the dome. The real reward, however, is the view outside, from the top of the dome. You get a 360 degree view out over Florence and the surrounding countryside.
4. Walk out of town, south of the Arno, to the hill known as Bellosguardo. Within half a mile of the Arno, the city stops and you will be surrounded by stone walls, olive groves and old farmhouses. When you finally get to the top of the hill, you can look back at the picture postcard view of the city, with Brunelleschi's great red tiled dome rising there in the middle of it.
Venice:
1. The famous tourist things:
a. Doge's Palace -- we got tickets for a special behind the scenes tour here.
Doge's Palace Secret Itineraries, Venice - Venice (Veneto) - Tickets and Booking It was quite fun. I've dealt with Select Italy many times and have always been pleased.
b. Basilica San Marco -- go here to get tickets to enter by the side door and skip the long line.
Tickets St Marks Basilica Venice Italy | Skip the Line San Marco Venezia
2. A little off the beaten path:
a. Museo Storico Navale -- The Italian Naval History Museum. It is incredibly cool and incredibly cheap. Heck, it's cheaper to pay to go in here and use their restroom than to go to one of the dedicated pay restrooms in town, plus you get to see all the cool displays. From Piazza San Marco, walk east along the Riva degli Schiavone to the Campo San Biagio, just past the Arsenale vaporetto stop.
Naval History Museum Venice - Marina Militare
b. after the Naval History Museum, continue east on the Riva degli Schiavone. You'll come to a large park, with real trees and grass - the only one we saw in Venice. In the park, right along the Riva, is the Ristorante Paradiso, which is a great place for lunch. You can sit on their terrace, look out over the lagoon and watch people walk by. You might see some of the midshipmen from the Naval Academy, which is at the very end of the park at the eastern tip of Venice.
c. Burano and Torcello -- we enjoyed these two islands more than Murano (we weren't all that interested in decorative glass). You can get to them by vaporetto (No. 12) from the Fondamente Nuove stop. The long boat ride out and back is most refreshing. I can recommend the Ristorante Riva Rosa on Burano for lunch.
Burano island of Venice Italy - Official Website and Guide
http://en.wikipedia.org/wiki/Torcello