there is an eruv in the ghetto. In fact you will need your passport to enter the italki shul.
your best bet is to contact Chabad of Venice for all your information. Gam Gam has wonderful food all week, and offers shabbos gratis. of course I have no doubt you will leave Rami a very generous donation. The other restarant in the square is overpriced for Shabbat meals, and the food leaves alot to be desired.
Have a wonderful time!
Jewish Education and Family Centre Chabad Berlin offers you a beautiful Synagogue, a kosher Restaurant, a Mikvah, a Judaika Store, a tourist centre and a Thora school. Experience the Jewish Life in Berlin in a warm atmosphere