Venice - amazing place - need a return visit here. The 4 hour backstreet walking tour was only about 2 hours and the 'refreshments' were insulting. Do one of the bigger tours or just get out and walk. Start in St. Marks Square and just walk - you'll soon get the hang of it. We walked for 7 hours. If you want to see the main places of interest be prepared to queue, but be aware that you can walk by yourself to the Grand Canal and see lots on the way. (Avoid Hard Rock Cafe exorbitant prices, but you can sneak in to use the loo).
Dubrovnik - fab place. If you're walking the walls then get there early as it can be baking hot and there are lots of steps, so avoid if you're not reasonable agile. Ticket office is through archway and round to the right - you'll need your credit card or local currency. Do wander down to the port and also take the cable car (about 10 Euros) to get a panoramic view.