A must is Nessebar.
Is situated directly opposite RCN across the bay. It is easily accessable by Sea or road. A boat leaves the new marina on a regular basis and you can catch a bus from the main road if you want to get there cheaply.
Nessebar originally a Thracian settlement known as Menebria was founded in the 2nd millennium BC. It is now a busy warren of ancient streets lined with very old traditional houses and little shops selling their wares.
There are many restaurants scattered around Nessebar so you can plan for a whole day there. Another must is to venture down to the marina area of Nessebar where it is possible to catch boat/catamaran rides. The fish restaurants in this area although a little pricey are great. T
Enjoy