Another longtime lurker here. I saw this thread, and decided to sign up so that I could put in my two cents.
I can’t tell you anything about festivals/events that might be going on, but if you read Italian and/or French, there are magazines with that sort of information: Roma C’e for Rome, and Zurban for Paris.
Rome is a city of churches. They’re filled with surprises - you can walk into a a small church that looks like it’s nothing special, and stumble across works of art by Renaissance and Baroque masters. Some of my favorites:
Santa Maria Sopra Minerva (near the Pantheon): Rome’s only true gothic church. Contains the Carafa Chapel, which was decorated by Filippino Lippi, and one of Michelangelo’s minor statues, (can’t remember its name off the top of my head.)
Santa Maria in Trastevere: A very old church. Simple, but beautiful. There’s also another church in the area, Santa Cecilia in Trastevere, that houses the Blessed Ludovica, an amazing Bernini sculpture. Trastevere itself is worth exploring too - it’s filled with cobblestone streets and unexpected courtyards. To get there, take the #8 tram from Largo Argentina, and get off at the stop right after you cross the bridge. (Trastevere = “across the Tiber.”) There’s an amazing bakery on Via del Moro called La Rinella - the pizza and the bread are delicious and cheap. The “brutti ma buoni” cookies are also nice if you like hazelnuts. There’s a restaurant on the same street, Mario’s, that does traditional Roman cuisine and is student-friendly. You can have a two-course meal with a glass of wine for under 20 euros.
San Giovanni in Laterno (metro stop Cavour): Pope Julius II’s tomb (Michelangelo) is located in this church. It’s an impressive work, with a fascinating history. The Captives (Dying Slave and the other one, whose name I can’t remember) that are in the Louvre were originally intended for the tomb.
Right near the Cavour metro station is “La Cigala e la Formica” (Via Leonina), a little restaurant that has what might be one of Rome’s best deals: a two-course “business lunch” for 10 euros. The menu’s only available in Italian, but the waitress does speak English. Also in the area is “La Carbonara dal 1906,” another restaurant serving traditional Roman cuisine. The specialty, as you can probably guess, is spaghetti alla carbonara. I don’t know how much English they speak, though.
I’ll second the poster who recommended eating gelato several times a day. I know the place near the Pantheon, and I agree with the recommendation, but I’ll add a few more to the list. Gelateria “Chocolat” (Via Leone IV, right near the Vatican) has flavors run to the unusual. Green tea, wasabi, and chocolate with pepperoncini (chili peppers.) Also near the Vatican is Millenium, which is unusual for its late hours (I think it’s open until 2am.) There’s also the Gelateria “Old Bridge” near Piazza Risorgimento, where the gelato is wonderfully creamy and the portions are huge. Via Ottaviano, which runs off Piazza Risorgimento, is also a good shopping street. There’s a store - I think it’s called Castroni - that deals in specialty and artisanal food products. It’s a great place for edible souvenirs - chocolate, wine, etc.
Also worth seeing in Rome are the markets. For a food market with plenty of character, go to Via Trionfale (a short walk from metro stop Cipro) anytime between 9am and 2pm, Monday through Saturday. If you feel like a picnic lunch, you can pick up all the fixings here. (You can take that picnic lunch to the Villa Borghese, the big park right by the Piazza del Popolo. Also located in the Villa Borghese is the Galleria Borghese, home to some of Bernini’s most stunning sculptures.) For bric-a-brac and antiques, there’s Porta Portese (bus or tram, your guidebook should have directions). I cannot stress this enough, however: if you go, don’t take anything of value with you. Carry cash only, and a small amount at that. The pickpockets are notorious.
Florence is an extremely compact, walkable city. I’d recommend reserving your tickets for the Uffizi online - if your time is limited, you really don’t want to be stuck in a queue for four hours. Especially not during the summer. If you don’t get enough of a Renaissance fix at the Uffizi, right near the train station is Santa Maria Novella, a church filled with Renaissance art.
Other things worth seeing include the Baptistry (the bronze doors are copies, though - I’ve forgotten which museum the originals are in), the Brancacci Chapel (check your guidebook for details), and the Duomo - I’m scared of heights, so I didn’t go up, but I’m told that the view’s amazing.
I loved Venice, but I went during the winter, and I’m told that the city has a slightly different character during the summer. First, a bit of advice for finding your way around. Figure out how to get from the place you’re staying to the train station. This is absolutely essential. Street names are pretty useless, but there are yellow signs with arrows everywhere, that point to two things: the train station and the Rialto bridge. As long as you can get to the train station, you’re fine.
The Piazza San Marco and the Basilica are both a must-see. Visit the Academia for Venetian art. Depending on how much you like Tintoretto, you may want to visit the Scuola di San Marco, which contains a room that has been described as “Tintoretto’s Sistine Chapel.” In all honesty, though, I’d just recommend wandering the streets - Venice is a fascinating city for aimless exploration. On the food front, try to avoid the places that advertise pizza in sixteen languages (easier said than done, I know) - see if you can find somewhere that does Venetian cuisine. Specialties include sarde in saour, sweet-sour sardines with pine nuts, squid cooked in its own ink, and dried cod (baccala) pureed with cream. To drink, go with prosecco (sparkling white wine). A very Venetian aperitif is the spritz, Campari with white wine.
I have suggestions for Paris, too, but I think I’d better split this into two posts.