The Etruscan necropolis at Cerveteri is half an hour east by train or car — vast, atmospheric, and a UNESCO site. Tarquinia is just up the coast and has the famous painted tombs, also UNESCO-listed. Rome is 80 minutes by train and obviously a full day rather than a half. Cruise passengers with one day in port nearly always choose Rome; we would suggest Cerveteri or Tarquinia instead, both far less crowded and arguably more interesting if you have spent time in Rome before.