There is a long pebbly public beach at Sveti Stefan, but it is worth noting that many of the beaches in the area are for the private use of hotel guests.
For example, the beaches either side of the causeway that leads to Sveti Stefan's famous islet are reserved for use of guests at the Aman Resort that occupies the islet. The beach is clearly marked as being reserved for hotel guests and, if you are in any doubt, the army of security guards keeping an eye on the beach will be quick to tell you that you're not allowed to proceed any further. Even on the day that we visited, when the rain was falling and the beaches were empty, the security staff could still be seen in their smart uniforms keeping a watchful eye on any would-be trespassers.
The situation is the same as you walk along the coast away from the islet. The wonderfully picturesque beaches of Kraljeva and Kraljicina are also reserved for guests of the nearby hotels.
Compared to the public beach, the private beaches tend to be sandier (but still mixed with pebbles) and provide rows of sun loungers for their guests. If you are in any doubt, check the signpost at the entrance to the beach; it will clearly state whether it is an "Organised Public Beach" or an "Organised Hotel Beach".
The cliffs might not be so tame, the island is on very high cliffs, quite steep, going directly to the sea, and it can be dangerous to seek fun climbing down, unless you're a professional alpinist :) and a rock climbing practicer. :)