Can also recommend the city wall walk in Dubrovnik, fascinating view down on to the town. In Venice try wandering away from the st marks square and in to the less popular side streets. You will find interesting little shops and the food and ice cream will be much cheaper than in the main tourist traps.
Dubrovnik is one of my favourite places, it's stunning.
We went on a cruise from Venice last year and we had a great time. I must say that departing from Venice can be a bit tricky, but once you make it to the boat, it's great. Venice itself is amazing, although it can be extremely busy and hot in August. We had young children with us, and it was not easy to navigate through the tiny passages full of tourists, but they sure remember it well. We spent most of the 8 hours walking around, and looking at the architecture. If you have any questions, let me know.
Also, Dubrovunick is wow!!!! A taxi that we got from the port (better deal as we are 5) took us to the old town and we walked around for three hours, then the same taxi met us just outside the old town and took us to a beach where he picked us up after 2 hours too as it was very virgin and nothing else to do. The people are very nice, although we did have a bit of communication problem.
We are headed to Venice in August and doing the Royal Carribean cruise anyone out there stayed in Venice that could give us tips on what to do and where to go? even if they did the cruise to Koper, Ravenna, Bari and Dubrovnik? help appreciated !!!!