Heading to Italy for the first time at the end

Heading to Italy for the first time at the end of April of this year.  Venice, Florence and Rome.  We are seeing most of the suggested sites.  Anything special that we should definately do or see?   Traveling by train to Florence and then to Rome.   Going to a food tour, and Tuscany for a wine tour.  Would also like to see the olive tree sites?   Just overwhelmed with all the things to do and see.  Any suggestions on food and where to eat??