Buon viaggio, m'amica. Of course I envy you two - on the other hand, I really enjoy hearing about other people's travels.
I don't have many warnings, but I'll try to post right now on them. On what I liked or loved, I'll try to be spare, which will be a challenge. Will post the greats on another post, or maybe more than one.
I've been riffled by zingari a few times. But I haven't gone at tourist season peaks and, at least on the first two trips, was visibly apparent as a tourist amid non-tourists. I don't know how prevalent such thefts are these days. (In Rome, I've read many times to watch your pockets on Bus Route #64). Anyway, for my first two trips I wore either a waistband type or neck hanging packet to keep my credit cards/i.d. and serious money in (I hated both, the waist band type somewhat less); on my last trip, I wore travel smith clothes that have "secret pockets" with zippers. (Or one can devise one's own secret pockets if handy with needle and thread.) This stupidity turned out to be useful on all three trips. I also carried a purse, but with nothing in it I wouldn't mind losing. I wasn't ever in a tour group, which I suppose is protective - don't know if you two will be.
I wouldn't be afraid re all this - don't have your stuff accessible and then don't worry about it. The only person I know to have been hurt by this is a friend who held on to her purse, and thus had her shoulder wrenched, in Firenze. So, that's another point - if it happens, let the purse go bye bye.
If you buy train tickets for Euro Star (the fast train), make sure to reserve the seats. I didn't have a clue this could matter, that a ticket might not mean "a seat by definition" and we stood from Rome to Bologna. That was in '99, may have changed. This may also relate re other train tickets, but other than that one occasion, I've always gotten a seat with no problem... on many trains.
Someone else on a2k may know more about this reserving/not reserving business.
If you make a trip to Ostia Antica (ruins close to Rome near the sea, in Ostia, and you decide to do an overnight, believe them when they tell you the hotels are in Ostia Lido, the next stop from Antica. We wasted time going back and forth. However, we liked Ostia Antica a lot, and the train trip from Rome is not very long - it's easily a day trip.
Long lines don't always mean slow. The mid-day line for gelato or sorbet at Giolitti in Rome - Giolitti, near piazza Montecitorio, is my favorite gelato/pastry/coffee place, hands down - goes very fast, as lines go. If there is a long line, though, they don't want you to tarry in decision making, and at the height of the day there are about sixty types of gelato or sorbets. So, for my first time there, or any popular gelato place, I'd try to go and check it out at some off hour, just to stare at the possibilities. Or maybe there is a list online - I'll see if I can find one.
As for calories, the dollops you get are smallish, full of flavor, and you can get tiny cups. They don't mind people who tarry re decisions when it isn't crowded, that I ever noticed.
In case you are asked when you don't have your passport with you (which can happen, as you leave it with the hotel desk at the beginning of a stay, and they return it) - keep a photo copy with your stuff.
On getting around - I'm guessing you two are walkers, which I highly recommend if one is able. Or, combining local transport and walking a lot.
However, your feet can start to kill you if you walk up to ten miles a day, as I've done. Thus, if you are walkers, bring really really really comfortable walking shoes/walking sandals.
Which brings up taxis, if you find yourself pooped. In Rome (I don't know the taxi situation in Firenze), there are some taxi stands, but not many. Standing and waving at a taxi is said to do you no good at all - you have to call for one, other than going to the taxi park areas (piazza del Populo is one, and there's one just north of piazza Navona, I've read; must be a list online).
Only time I've taken a taxi in Rome, except for one airport trip, was when I had a lonnnnnng day getting to Rome from Viterbo, and hopped a cab at pz. del Populo. It didn't cost that much (then) to cross town to the Aventine hill area where I was staying. (Prices may vary..)
If you take a cab from your hotel to the airport, have the desk clerk ascertain and set the amount of money for you - these can be higher money and people get overcharged. Same for taxis from Termini - prices may vary. From Termini, I've walked or used the Metro instead.
Luggage - less is more. We learned this the hard way. I would consider including two odd things, depending on your needs - one is a cheapo sort of sports duffle that you can fold up when packing and use for return trip if you buy stuff. You can get those there from street hawkers, but we wasted time looking on our first trip. Another - depends on the places you stay - could be one or two plastic hangers.. if only for washing out things in the shower/bathroom. Or, I suppose, one of those travel clotheslines, which I've had but never used.
Sorry for putting the quandary stuff first. More later on delights/restaurants/museums.