Anything from Caputo flour to truffles, mozzarello or ham...
Make sure you check the U.S. Customs site before you get to excited about fresh/dried foods. A hint that can save some time: Be sure to declare every food item (the form on the plane), then ask to go thru the agricultural inspection site at Customs when reentering the States. Tell them you are not sure about something you have. The agricultural inspection stations are MUCH faster than the normal customs. At least that has been my experience.
Any hard-to-source items that I should be looking at picking up?
I like Mostarda, and very few types are available in the States. I always bring back a few jars of unusual ones I find, for myself, and for gifts.
Have fun.......the fall is the best time to visit.