I’ve only been to Dubrovnik but its very nice and well worth a visit, we stayed there for a few days and there was enough to do although we had a few day trips away too. Dubrovnik gets really busy between 1100 and 1600 when the cuise liners dump passengers off and they wander through the town in groups of 100+, the town depends on the tourist market so they are treated with many false smiles. the place is transformed in the later afternoon/evening though when the locals come out and if you are still there in the evening, you get much better service as they work out that you are staying local and not on a cruise!
The walk from Dubrovnik, through Lapad and then round the headland and back to the main port is nice, this is where some of the liners dock, others just yuse tenders and park outside Dubrovnik harbour. Take a boat out to Lokrum too and have a wander round the island. The beaches are rocky but there are enough flat bits for it not to be an issue.
we got the scheduled jadralina passenger ferry to the islands of Mljet and Lopud. Lopud has an amazing beach and is wel lworth a trip, Mljet is a nature reserve and has some nice walks through forests. You can probably hire bikes on Mljet, swimming in the sea is easily possible upto late September, there are lots of little fish who will try to eat the hairs on your legs though which can be fun/terryifying. they are big on sea kayaking and that sort of thing too. we went with hidden croatia who were fantastic and stayed in the hotel komodor.
Croats are a friendly bunch. we had a great guide book written by a local, i’ll find it tonight and update the thread, it had some good recommendations to keep you off the tourist trail.