It depends on where we are going to hike.
Most of the time, in the Dolomites we pass through several huts ("rifugio") where there are proper restaurants and bars. In this case you only need water and a snack (bars or nuts or whatever you prefer).
For tours on Lake Garda we recommend you bring a sandwich/full lunch and a snack because it is possible that we do not pass through any "rifugio".
For multi-day tours on the Amalfi Coast, Islands and Tuscany, your guide will tell you every day what you need for the next day depending on the chosen itinerary.