Haven't been to Venice. You might want to get passes for the Hop on/Hop off bus in Barcelona. Then you can get on an off to sight see where ever you want. La Rambla street is amazing. You can get a nice Tapa lunch for around 10 Euro. La Rambla is close to a mile long and is full of arts & craft stands, mimes, flower carts, and even merchants selling birds and small pets.
Welcome to cruise-chat.