Miami and Southern California have the best beaches in the continental United States, though they all pale in comparison to the spectacular sands on all the islands of Hawaii.
The entire Atlantic is lined with sand where you can sun and swim in the summer, and you'll find no shortage of resorts and beach motels. If you try hard enough, you can even find a little undeveloped solitude at the Cape Cod National Seashore near Provincetown, Massachusetts, and at Cape Hatteras National Seashore on North Carolina's Outer Banks.
The Maine coast is gorgeous but too cold for actual swimming. The same goes for the lovely, dramatic scenery in Northern California and along the Oregon coast.