We will be in several Italian cities in May and will need to charge batteries for a cell phone and camera. Both chargers read "AC100-240 V" for input. What will I need for an adapter and converter?
You will only need an adapter, to make sure U-S plugs fit Italian outlets. Your chargers will automatically adjust for the different electricity voltage, so you do not need a converter.
I also take an extension cord since rooms do not always have convenient outlets; you'll need a converter + plug for this.
What you want is commonly called a "plug adapter". It has two round pins to go into the wall and accepts U.S. two bladed plugs. They should fit into receptacles all over the European continent. If you have a choice, get the adapter for So. Europe. The pins are 4.0mm diameter. The ones for No. Europe are 4.8mm and won't fit into Italian receptacles.
I can't say about Italy, but I have spent 70 night in Germany recently, and I never needed an extension cord. The line cords for my chargers are long enough.
What you might need is one of those three-to-one adapters like we use in the U.S to expand the capacity of wall receptacles. That way you could charge your phone and camera at the same time. Be sure to tape the 3-1 adapter to the plug adapter, because the European receptacles tend to grip those round pins tightly, and you might inadvertantly leave the adapter in the wall. (I did that once.)