we had 5 days there a few years ago (2014?) and we're itching to go back, we saw most of the main stuff but would do so again. We stayed in an area called Monte, close to the Forum. If you keep off the main tourist areas to eat, we found it very reasonable, Piazza Novona for example is very expensive so we only had a couple of drinks there walking back from the Vatican, really just to say we'd been and spend an hour or so watching the street art/theatre there
Its very much a walking city so wear comfy shoes. Mrs fufkin wore sandals the first day which ended up in her bag as we had to buy her some trainers! Food is brilliant, we never had a poor meal and you can easily have a meal and drink each for less than €30, if as I say you get off the beaten track a bit. Monte was great for this as the restaurants were full of locals, we didn't bump into many tourists eating in this area. Lunch was usually a salad as it was so hot, then pizza or pasta for an evening meal
We booked the Vatican in advance for a guided tour, the rest we just turned up for and queues were OK, this was in September. Buy a little guide book with a map to help navigate as it can be a little confusing working out where everything is in relation to each other, we got the rough guide pocket book which was great
When it come to art, don't just go to the main churches, go in every one you stumble upon, even these are full of amazing prices. Make sure the Mrs takes a shawl or wrap of some sort as its very much frowned upon for women to enter with bear arms
Keep your wits about you, like anywhere and you'll be fine, don't have bags or phones on show as they whizz past on scooters and whip stuff out of your hand. I have a cross body bag which might make me look a tīt, but I didn't lose any belongings, and be careful in the Pantheon because while you're looking up, someone is going through your bag or pockets, its free to get in so is the worst place for pick pockets
). The food is amazing and smashed in my own body weight in pasta and ice cream. 
