• Places to have a bite to eat: in Venice it is very typical to eat "cicchetti", small portions of traditional Venetian food. San Polo has some great cicchett bars, like the Cantina Do Spade in Do Spade street (try the tuna polpette and the sardines with onion), the Cantina do Mori (go for the melanzane, the sardines, the fresh anchovies and the sandwiches) or the Osteria al Sacro e Profano on the street that runs parallel to the colonnades that go to Rialto (really good cod and toast with tomato). 
• Places for lunch and dinner: Trattoria Rialto Novo (in the Campiello Rialto Nuovo), where there are good pasta dishes with fish (15-20€ per person). Although if you prefer to eat with views to the bridge (and don't care about lower value for money), then go to the Ristorante Terrazza Sommariva, on Riva del Vin street. The food is for tourists (good pasta, small portions, high prices...) but the views are fine. 20€ per person. You'll find better food on Riva Rialto resturant, but they don't have terrace and getting a table with views is difficult.