I spent a night in Orebić when visiting Korčula, having reached it by catching a boat from the latter's city centre. I remember it as a pleasant and peaceful town. There is not much to see in Orebić; it is really more the tranquil, relaxed nature of the place that makes it an attractive town to visit, especially if you have already been to cities such as Split and Dubrovnik beforehand.
There are nice views across to the island of Korčula from Orebić, and the stroll along the seafront is pleasant too. If want to explore a little local history while you're here, I recommend a walk to the Franciscan monastery located at the edge of the town, which makes for an enjoyable place to visit.
I remember eating in a nice restaurant on the seafront at one point, and ending up chatting to some of the locals, who were pleased to see tourists stopping to visit their town.
As far as beaches are concerned, the best ones are to be found on the peninsula. So it is a good idea to have a car available so you can easily travel the few miles required to reach them … and have the opportunity to enjoy some well-deserved rest and relaxation!