JDiver, you are lucky to be spending so long in Dubrovnik!
There is a 3 island boat trip/Fish Picnic that leaves 9am and returns 6pm. It includes lunch and drinks and is not expensive (£43/$69/352 kuna). It sounds like a wonderful day out.

Sadly, our ship departs at 6pm so we can't participate. We are in Dubrovnik from 8am-6pm, though we may not disembark until mid morning depending on how our cruise line arranges this. We are also travelling with a 74 year old so will be limited to what we can do, we even think the walls may be out of the question!
Any suggestions for us?