Rincon has BEAUTIFUL beaches, all with their own personality. A few that we visited and loved:
Domes Beach and Spanish Wall Beach - connected by a trail near the old power plant. The beaches are popular with surfers but during the day are most deserted. Spanish Wall is hidden; we never saw anyone either time we were there.
Steps Beach - Has small tidal pools that capture neat aquatic life to watch. Steps Beach is named for the concrete stairs that sit in the shallow water. It's more popular with families but it still a beautiful beach.
Pools Beach - Towards the north side of Rincon, it's a wide, flat beach with beautiful rock formations on either side. It was abandoned when we went but I've heard it gets families on the weekends.