Running barefoot on the beach, either near the surf or up on the dry loose sand, is a great workout! It's intense, requires more energy, strengthens the lower legs and feet, as it allows them to move naturally through the motion, and has less impact forces on your body.
The best time to run near the surf is when the tide is going out (about 2-3 hours before low tide), when it's most hard-packed and level. If you run when the tide is coming in, it will just be a slow slog of a run, as your feet sink into the water-soaked sand.
Make sure you don't do too much too soon. Start with 15-20 min and add more gradually as you get used to it.