If you are spending some days in Venice you will probably have time to visit Padua also.
Padua is a very nice city just 30 kilometers from Venice and easy to be reached by several trains (travel about 40 minutes). Hisorical town with a very ancient university. One day visit must include Prato della Valle and St. Antony Church, Giotto’s masterpiece at the Cappella degli Scrovegni, a walk in the city centre with the university building (Bò), the beautiful squares (piazza delle Frutta, Erbe, Signori, San Nicolò, Cavour) and the main streets including the ancient Ghetto. This area you will find many nice restaurants. Also suggested a break in the fantastic Caffè Pedrocchi (also for lunch).