I have two favorite beach spots:
Rehoboth Beach, DE, because it has a wonderful boardwalk and a revitalized main street with lots of shops, restaurants and free seasonal entertainment at the new bandstand which overlooks the ocean.
Hilton Head Island, SC beaches, especially Coligny Beach because it has wide clean, hard packed sand which makes bike riding directly on the beach an awe-inspiring activity. Coligny beach has great facilities including colorful wooden rocking chairs, wooden swings, benches and synchronized water fountains that visitors can run through and cool off in. There is also a nearby Tiki Hut and bar which has volleyball nets set up. The island also has many fine restaurants and shops and miles of paved multiuse trails that are perfect for biking, walking, jogging and rollerblading.