As it happens I've just come back. Myself, wife, and 2 year old.
We stayed on an island near Dubrovnik called Sipan which was very nice, but perhaps slightly too quiet.
If it's beaches you're after then I can recommend Lopud, another island in the same area. We went there on a day trip, and it's got a lovely very sandy beach. It's about 45 minutes from Dubrovnik on the boat. Bear in mind that the boats are funny times; there's on mega early, then no more until midday.
Dubrovnik itself is very nice, really interesting and lovely old buildings.
I've also heard that Cavtat is nice too.
Try and go scuba diving, or at least snorkelling - the sea is beautifully clear.