Socks. (Or tights if you are wearing dresses.) When you're doing a lot of walking, and may be dealing with wet feet/shoes, having a spare pair of socks can really help.
Does your camera use a special battery / memory card? You didn't list charger/spares. I take LOTS of pictures, so that's always high on my list.
I'll second the "it may be colder than you think" posters above. I've frozen in Venice in May - and the breeze off the water can be wicked cold! Of course two days later I was in Florence, the sun came out & I was in short-sleeves.
I also include sunglasses, passport, plane tickets, hotel reservations, and credit/ATM cards as "major essentials" on my packing list.
