Personally I think you might want more than two nights in Dubrovnik - it is a great place to explore or just hang out eating and drinking. Maybe 3 nights?

On the island front I would recommend Mljet with its national park.

As always with this part of the world, check out the In Your Pocket guide. In addition the National Tourist Board web site has a lot of ideas and is very pretty!!

Enjoy Croatia, it is one of my favourite holiday destinations.