I would vote for Naples and Amalfi coast for its mild weather. Anyway also Venice it is fine if you use Venice (or Padua) as a base to visit the region (Padua, Verona, Trento, Vicenza and the wine area Colli Euganei). Anyway, take into account 2 things: 1- April is a quite rainy month, and 2- April is a busy month because many Italians take holidays because Easter week and, moreover, April 25th and May 1st are bank holiday in Italy so that be sure to book well in advance for that season!!!

Ciao, Marco in Milan - Italy