2,5 hours - Fall in love with the sestier de Cannaregio. Hidden corners, an unexpected park, the Jewish Ghetto (if you want a deep view just of the ghetto I have a special tour), bacari, the Merchant of Venice area, the sites where the painters Tintoretto and Tiziano used to live, and the Venetian pantheon!
2 hours - my top tour of the Jewish Ghetto of Venice